diff --git a/README.md b/README.md
index f5a4c81..ad968b9 100644
--- a/README.md
+++ b/README.md
@@ -28,6 +28,8 @@ Implementation of various data structures and algorithms in Go.
     - [Iterator](#iterator)
       - [IteratorWithIndex](#iteratorwithindex)
       - [IteratorWithKey](#iteratorwithkey)
+      - [ReverseIteratorWithIndex](#reverseiteratorwithindex)
+      - [ReverseIteratorWithKey](#reverseiteratorwithkey)
     - [Enumerable](#enumerable)
       - [EnumerableWithIndex](#enumerablewithindex)
       - [EnumerableWithKey](#enumerablewithkey)
@@ -53,17 +55,18 @@ Containers are either ordered or unordered. All ordered containers provide [stat
 
 | Container | Ordered | [Iterator](#iterator) | [Enumerable](#enumerable) | Ordered by |
 | :--- | :---: | :---: | :---: | :---: |
-| [ArrayList](#arraylist) | yes | yes | yes | index |
+| [ArrayList](#arraylist) | yes | yes* | yes | index |
 | [SinglyLinkedList](#singlylinkedlist) | yes | yes | yes | index |
-| [DoublyLinkedList](#doublylinkedlist) | yes | yes | yes | index |
+| [DoublyLinkedList](#doublylinkedlist) | yes | yes* | yes | index |
 | [HashSet](#hashset) | no | no | no | index |
-| [TreeSet](#treeset) | yes | yes | yes | index |
+| [TreeSet](#treeset) | yes | yes* | yes | index |
 | [LinkedListStack](#linkedliststack) | yes | yes | no | index |
-| [ArrayStack](#arraystack) | yes | yes | no | index |
+| [ArrayStack](#arraystack) | yes | yes* | no | index |
 | [HashMap](#hashmap) | no | no | no | key |
-| [TreeMap](#treemap) | yes | yes | yes | key |
-| [RedBlackTree](#redblacktree) | yes | yes | no | key |
-| [BinaryHeap](#binaryheap) | yes | yes | no | index |
+| [TreeMap](#treemap) | yes | yes* | yes | key |
+| [RedBlackTree](#redblacktree) | yes | yes* | no | key |
+| [BinaryHeap](#binaryheap) | yes | yes* | no | index |
+|  |  | <sub><sup>*reversible</sup></sub> |  |  |
 
 ### Lists
 
@@ -572,12 +575,12 @@ Some data structures (e.g. TreeMap, TreeSet) require a comparator function to au
 
 Comparator is defined as:
 
-Return values:
+Return values (int):
 
 ```go
--1, if a < b
- 0, if a == b
- 1, if a > b
+negative , if a < b
+zero     , if a == b
+positive , if a > b
 ```
 
 Comparator signature:
@@ -642,7 +645,7 @@ func main() {
 
 ### Iterator
 
-All ordered containers have stateful iterators. Typically an iterator is obtained by _Iterator()_ function of an ordered container. Once obtained, iterator's _Next()_ function moves the iterator to the next element and returns true if there was a next element. If there was an element, then element's can be obtained by iterator's _Value()_ function. Depending on the ordering type, it's position can be obtained by iterator's _Index()_ or _Key()_ functions.
+All ordered containers have stateful iterators. Typically an iterator is obtained by _Iterator()_ function of an ordered container. Once obtained, iterator's _Next()_ function moves the iterator to the next element and returns true if there was a next element. If there was an element, then element's can be obtained by iterator's _Value()_ function. Depending on the ordering type, it's position can be obtained by iterator's _Index()_ or _Key()_ functions. Some containers even provide reversible iterators, essentially the same, but provide another extra _Prev()_ function that moves the iterator to the previous element and returns true if there was a previous element.
 
 #### IteratorWithIndex
 
@@ -668,6 +671,32 @@ for it.Next() {
 }
 ```
 
+#### ReverseIteratorWithIndex
+
+A [iterator](#iterator) whose elements are referenced by an index. Typical usage:
+
+```go
+it := list.Iterator()
+for it.Next() { /* Move to end */ }
+for it.Prev() {
+	index, value := it.Index(), it.Value()
+    ...
+}
+```
+
+#### ReverseIteratorWithKey
+
+A [iterator](#iterator) whose elements are referenced by a key. Typical usage:
+
+```go
+it := map.Iterator()
+for it.Next() { /* Move to end */ }
+for it.Prev() {
+	key, value := it.Key(), it.Value()
+    ...
+}
+```
+
 ### Enumerable
 
 Enumerable functions for ordered containers that implement [EnumerableWithIndex](#enumerablewithindex) or [EnumerableWithKey](#enumerablewithkey) interfaces.
diff --git a/containers/containers.go b/containers/containers.go
index 0a26c44..6e73dea 100644
--- a/containers/containers.go
+++ b/containers/containers.go
@@ -24,11 +24,18 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
+// Package containers provides core interfaces and functions for data structures.
+//
+// Container is the base interface for all data structures to implement.
+//
+// Iterators provide stateful iterators.
+//
+// Enumerable provides Ruby inspired (each, select, map, find, any?, etc.) container functions.
 package containers
 
 import "github.com/emirpasic/gods/utils"
 
-// Container is base interface that all data structures implement
+// Container is base interface that all data structures implement.
 type Container interface {
 	Empty() bool
 	Size() int
diff --git a/containers/enumerable.go b/containers/enumerable.go
index 0a09207..027de5a 100644
--- a/containers/enumerable.go
+++ b/containers/enumerable.go
@@ -24,9 +24,6 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Enumerable functions for ordered containers.
-// Ruby's enumerable inspired package.
-
 package containers
 
 // EnumerableWithIndex provides functions for ordered containers whose values can be fetched by an index.
diff --git a/containers/iterator.go b/containers/iterator.go
index b3d2254..c1a7c54 100644
--- a/containers/iterator.go
+++ b/containers/iterator.go
@@ -24,8 +24,6 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Stateful iterator pattern for ordered containers.
-
 package containers
 
 // IteratorWithIndex is stateful iterator for ordered containers whose values can be fetched by an index.
@@ -55,3 +53,33 @@ type IteratorWithKey interface {
 	// Does not modify the state of the iterator.
 	Key() interface{}
 }
+
+// ReverseIteratorWithIndex is stateful iterator for ordered containers whose values can be fetched by an index.
+//
+// Essentially it is the same as IteratorWithIndex, but provides additional Prev() function to enable traversal in reverse.
+type ReverseIteratorWithIndex interface {
+	// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+	// If Prev() returns true, then previous element's index and value can be retrieved by Index() and Value().
+	// Modifies the state of the iterator.
+	Prev() bool
+
+	IteratorWithIndex
+	// Next() bool
+	// Value() interface{}
+	// Index() int
+}
+
+// ReverseIteratorWithKey is a stateful iterator for ordered containers whose elements are key value pairs.
+//
+// Essentially it is the same as IteratorWithKey, but provides additional Prev() function to enable traversal in reverse.
+type ReverseIteratorWithKey interface {
+	// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+	// If Prev() returns true, then previous element's index and value can be retrieved by Key() and Value().
+	// Modifies the state of the iterator.
+	Prev() bool
+
+	IteratorWithKey
+	// Next() bool
+	// Value() interface{}
+	// Key() interface{}
+}
diff --git a/lists/arraylist/arraylist.go b/lists/arraylist/arraylist.go
index f471908..c08d928 100644
--- a/lists/arraylist/arraylist.go
+++ b/lists/arraylist/arraylist.go
@@ -24,10 +24,11 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of list using a slice.
+// Package arraylist implements the array list.
+//
 // Structure is not thread safe.
-// References: http://en.wikipedia.org/wiki/List_%28abstract_data_type%29
-
+//
+// Reference: https://en.wikipedia.org/wiki/List_%28abstract_data_type%29
 package arraylist
 
 import (
@@ -41,7 +42,7 @@ import (
 func assertInterfaceImplementation() {
 	var _ lists.List = (*List)(nil)
 	var _ containers.EnumerableWithIndex = (*List)(nil)
-	var _ containers.IteratorWithIndex = (*Iterator)(nil)
+	var _ containers.ReverseIteratorWithIndex = (*Iterator)(nil)
 }
 
 // List holds the elements in a slice
@@ -194,7 +195,19 @@ func (list *List) Iterator() Iterator {
 // If Next() returns true, then next element's index and value can be retrieved by Index() and Value().
 // Modifies the state of the iterator.
 func (iterator *Iterator) Next() bool {
-	iterator.index++
+	if iterator.index < iterator.list.size {
+		iterator.index++
+	}
+	return iterator.list.withinRange(iterator.index)
+}
+
+// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+// If Prev() returns true, then previous element's index and value can be retrieved by Index() and Value().
+// Modifies the state of the iterator.
+func (iterator *Iterator) Prev() bool {
+	if iterator.index >= 0 {
+		iterator.index--
+	}
 	return iterator.list.withinRange(iterator.index)
 }
 
diff --git a/lists/arraylist/arraylist_test.go b/lists/arraylist/arraylist_test.go
index 0dea8d8..9de67da 100644
--- a/lists/arraylist/arraylist_test.go
+++ b/lists/arraylist/arraylist_test.go
@@ -299,11 +299,21 @@ func TestListChaining(t *testing.T) {
 	}
 }
 
-func TestListIterator(t *testing.T) {
+func TestListIteratorNextOnEmpty(t *testing.T) {
+	list := New()
+	it := list.Iterator()
+	for it.Next() {
+		t.Errorf("Shouldn't iterate on empty list")
+	}
+}
+
+func TestListIteratorNext(t *testing.T) {
 	list := New()
 	list.Add("a", "b", "c")
 	it := list.Iterator()
+	count := 0
 	for it.Next() {
+		count++
 		index := it.Index()
 		value := it.Value()
 		switch index {
@@ -323,13 +333,52 @@ func TestListIterator(t *testing.T) {
 			t.Errorf("Too many")
 		}
 	}
-	list.Clear()
-	it = list.Iterator()
-	for it.Next() {
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
+	}
+}
+
+func TestListIteratorPrevOnEmpty(t *testing.T) {
+	list := New()
+	it := list.Iterator()
+	for it.Prev() {
 		t.Errorf("Shouldn't iterate on empty list")
 	}
 }
 
+func TestListIteratorPrev(t *testing.T) {
+	list := New()
+	list.Add("a", "b", "c")
+	it := list.Iterator()
+	for it.Next() {
+	}
+	count := 0
+	for it.Prev() {
+		count++
+		index := it.Index()
+		value := it.Value()
+		switch index {
+		case 0:
+			if actualValue, expectedValue := value, "a"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 1:
+			if actualValue, expectedValue := value, "b"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 2:
+			if actualValue, expectedValue := value, "c"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		default:
+			t.Errorf("Too many")
+		}
+	}
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
+	}
+}
+
 func BenchmarkList(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		list := New()
diff --git a/lists/doublylinkedlist/doublylinkedlist.go b/lists/doublylinkedlist/doublylinkedlist.go
index 58a0e4b..cb7bdfc 100644
--- a/lists/doublylinkedlist/doublylinkedlist.go
+++ b/lists/doublylinkedlist/doublylinkedlist.go
@@ -24,10 +24,11 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of doubly linked list.
+// Package doublylinkedlist implements the doubly-linked list.
+//
 // Structure is not thread safe.
-// References: http://en.wikipedia.org/wiki/Doubly_linked_list
-
+//
+// Reference: https://en.wikipedia.org/wiki/List_%28abstract_data_type%29
 package doublylinkedlist
 
 import (
@@ -41,7 +42,7 @@ import (
 func assertInterfaceImplementation() {
 	var _ lists.List = (*List)(nil)
 	var _ containers.EnumerableWithIndex = (*List)(nil)
-	var _ containers.IteratorWithIndex = (*Iterator)(nil)
+	var _ containers.ReverseIteratorWithIndex = (*Iterator)(nil)
 }
 
 // List holds the elements, where each element points to the next and previous element
@@ -315,12 +316,14 @@ func (list *List) Iterator() Iterator {
 // If Next() returns true, then next element's index and value can be retrieved by Index() and Value().
 // Modifies the state of the iterator.
 func (iterator *Iterator) Next() bool {
-	iterator.index++
+	if iterator.index < iterator.list.size {
+		iterator.index++
+	}
 	if !iterator.list.withinRange(iterator.index) {
 		iterator.element = nil
 		return false
 	}
-	if iterator.element != nil {
+	if iterator.index != 0 {
 		iterator.element = iterator.element.next
 	} else {
 		iterator.element = iterator.list.first
@@ -328,6 +331,25 @@ func (iterator *Iterator) Next() bool {
 	return true
 }
 
+// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+// If Prev() returns true, then previous element's index and value can be retrieved by Index() and Value().
+// Modifies the state of the iterator.
+func (iterator *Iterator) Prev() bool {
+	if iterator.index >= 0 {
+		iterator.index--
+	}
+	if !iterator.list.withinRange(iterator.index) {
+		iterator.element = nil
+		return false
+	}
+	if iterator.index == iterator.list.size-1 {
+		iterator.element = iterator.list.last
+	} else {
+		iterator.element = iterator.element.prev
+	}
+	return iterator.list.withinRange(iterator.index)
+}
+
 // Value returns the current element's value.
 // Does not modify the state of the iterator.
 func (iterator *Iterator) Value() interface{} {
diff --git a/lists/doublylinkedlist/doublylinkedlist_test.go b/lists/doublylinkedlist/doublylinkedlist_test.go
index e199102..1ff73b6 100644
--- a/lists/doublylinkedlist/doublylinkedlist_test.go
+++ b/lists/doublylinkedlist/doublylinkedlist_test.go
@@ -299,11 +299,21 @@ func TestListChaining(t *testing.T) {
 	}
 }
 
-func TestListIterator(t *testing.T) {
+func TestListIteratorNextOnEmpty(t *testing.T) {
+	list := New()
+	it := list.Iterator()
+	for it.Next() {
+		t.Errorf("Shouldn't iterate on empty list")
+	}
+}
+
+func TestListIteratorNext(t *testing.T) {
 	list := New()
 	list.Add("a", "b", "c")
 	it := list.Iterator()
+	count := 0
 	for it.Next() {
+		count++
 		index := it.Index()
 		value := it.Value()
 		switch index {
@@ -323,13 +333,52 @@ func TestListIterator(t *testing.T) {
 			t.Errorf("Too many")
 		}
 	}
-	list.Clear()
-	it = list.Iterator()
-	for it.Next() {
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
+	}
+}
+
+func TestListIteratorPrevOnEmpty(t *testing.T) {
+	list := New()
+	it := list.Iterator()
+	for it.Prev() {
 		t.Errorf("Shouldn't iterate on empty list")
 	}
 }
 
+func TestListIteratorPrev(t *testing.T) {
+	list := New()
+	list.Add("a", "b", "c")
+	it := list.Iterator()
+	for it.Next() {
+	}
+	count := 0
+	for it.Prev() {
+		count++
+		index := it.Index()
+		value := it.Value()
+		switch index {
+		case 0:
+			if actualValue, expectedValue := value, "a"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 1:
+			if actualValue, expectedValue := value, "b"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 2:
+			if actualValue, expectedValue := value, "c"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		default:
+			t.Errorf("Too many")
+		}
+	}
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
+	}
+}
+
 func BenchmarkList(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		list := New()
diff --git a/lists/lists.go b/lists/lists.go
index ad241c2..b3d9b9a 100644
--- a/lists/lists.go
+++ b/lists/lists.go
@@ -16,6 +16,11 @@ License along with this library. See the file LICENSE included
 with this distribution for more information.
 */
 
+// Package lists provides an abstract List interface.
+//
+// In computer science, a list or sequence is an abstract data type that represents an ordered sequence of values, where the same value may occur more than once. An instance of a list is a computer representation of the mathematical concept of a finite sequence; the (potentially) infinite analog of a list is a stream.  Lists are a basic example of containers, as they contain other values. If the same value occurs multiple times, each occurrence is considered a distinct item.
+//
+// Reference: https://en.wikipedia.org/wiki/List_%28abstract_data_type%29
 package lists
 
 import (
diff --git a/lists/singlylinkedlist/singlylinkedlist.go b/lists/singlylinkedlist/singlylinkedlist.go
index baf028c..f65f728 100644
--- a/lists/singlylinkedlist/singlylinkedlist.go
+++ b/lists/singlylinkedlist/singlylinkedlist.go
@@ -24,10 +24,11 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of doubly linked list.
+// Package singlylinkedlist implements the singly-linked list.
+//
 // Structure is not thread safe.
-// References: http://en.wikipedia.org/wiki/Linked_list#Singly_linked_list
-
+//
+// Reference: https://en.wikipedia.org/wiki/List_%28abstract_data_type%29
 package singlylinkedlist
 
 import (
@@ -287,15 +288,17 @@ func (list *List) Iterator() Iterator {
 // If Next() returns true, then next element's index and value can be retrieved by Index() and Value().
 // Modifies the state of the iterator.
 func (iterator *Iterator) Next() bool {
-	iterator.index++
+	if iterator.index < iterator.list.size {
+		iterator.index++
+	}
 	if !iterator.list.withinRange(iterator.index) {
 		iterator.element = nil
 		return false
 	}
-	if iterator.element != nil {
-		iterator.element = iterator.element.next
-	} else {
+	if iterator.index == 0 {
 		iterator.element = iterator.list.first
+	} else {
+		iterator.element = iterator.element.next
 	}
 	return true
 }
diff --git a/lists/singlylinkedlist/singlylinkedlist_test.go b/lists/singlylinkedlist/singlylinkedlist_test.go
index c546217..53c2ad8 100644
--- a/lists/singlylinkedlist/singlylinkedlist_test.go
+++ b/lists/singlylinkedlist/singlylinkedlist_test.go
@@ -299,11 +299,21 @@ func TestListChaining(t *testing.T) {
 	}
 }
 
-func TestListIterator(t *testing.T) {
+func TestListIteratorNextOnEmpty(t *testing.T) {
+	list := New()
+	it := list.Iterator()
+	for it.Next() {
+		t.Errorf("Shouldn't iterate on empty list")
+	}
+}
+
+func TestListIteratorNext(t *testing.T) {
 	list := New()
 	list.Add("a", "b", "c")
 	it := list.Iterator()
+	count := 0
 	for it.Next() {
+		count++
 		index := it.Index()
 		value := it.Value()
 		switch index {
@@ -323,10 +333,8 @@ func TestListIterator(t *testing.T) {
 			t.Errorf("Too many")
 		}
 	}
-	list.Clear()
-	it = list.Iterator()
-	for it.Next() {
-		t.Errorf("Shouldn't iterate on empty list")
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
 	}
 }
 
diff --git a/maps/hashmap/hashmap.go b/maps/hashmap/hashmap.go
index 6ce3145..6cb8565 100644
--- a/maps/hashmap/hashmap.go
+++ b/maps/hashmap/hashmap.go
@@ -24,11 +24,13 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of unorder map backed by a hash table.
+// Package hashmap implements a map backed by a hash table.
+//
 // Elements are unordered in the map.
+//
 // Structure is not thread safe.
-// References: http://en.wikipedia.org/wiki/Associative_array
-
+//
+// Reference: http://en.wikipedia.org/wiki/Associative_array
 package hashmap
 
 import (
diff --git a/maps/maps.go b/maps/maps.go
index 2e82434..93f8bc0 100644
--- a/maps/maps.go
+++ b/maps/maps.go
@@ -24,6 +24,17 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
+// Package maps provides an abstract Map interface.
+//
+// In computer science, an associative array, map, symbol table, or dictionary is an abstract data type composed of a collection of (key, value) pairs, such that each possible key appears just once in the collection.
+//
+// Operations associated with this data type allow:
+// - the addition of a pair to the collection
+// - the removal of a pair from the collection
+// - the modification of an existing pair
+// - the lookup of a value associated with a particular key
+//
+// Reference: https://en.wikipedia.org/wiki/Associative_array
 package maps
 
 import "github.com/emirpasic/gods/containers"
diff --git a/maps/treemap/treemap.go b/maps/treemap/treemap.go
index 4b8558d..f1640bf 100644
--- a/maps/treemap/treemap.go
+++ b/maps/treemap/treemap.go
@@ -24,11 +24,13 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of order map backed by red-black tree.
+// Package treemap implements a map backed by red-black tree.
+//
 // Elements are ordered by key in the map.
+//
 // Structure is not thread safe.
-// References: http://en.wikipedia.org/wiki/Associative_array
-
+//
+// Reference: http://en.wikipedia.org/wiki/Associative_array
 package treemap
 
 import (
@@ -41,7 +43,7 @@ import (
 func assertInterfaceImplementation() {
 	var _ maps.Map = (*Map)(nil)
 	var _ containers.EnumerableWithKey = (*Map)(nil)
-	var _ containers.IteratorWithKey = (*Iterator)(nil)
+	var _ containers.ReverseIteratorWithKey = (*Iterator)(nil)
 }
 
 // Map holds the elements in a red-black tree
@@ -143,6 +145,13 @@ func (iterator *Iterator) Next() bool {
 	return iterator.iterator.Next()
 }
 
+// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+// If Prev() returns true, then previous element's index and value can be retrieved by Key() and Value().
+// Modifies the state of the iterator.
+func (iterator *Iterator) Prev() bool {
+	return iterator.iterator.Prev()
+}
+
 // Value returns the current element's value.
 // Does not modify the state of the iterator.
 func (iterator *Iterator) Value() interface{} {
diff --git a/maps/treemap/treemap_test.go b/maps/treemap/treemap_test.go
index c081431..ac4d778 100644
--- a/maps/treemap/treemap_test.go
+++ b/maps/treemap/treemap_test.go
@@ -311,14 +311,34 @@ func TestMapChaining(t *testing.T) {
 	}
 }
 
-func TestMapIterator(t *testing.T) {
+func TestMapIteratorNextOnEmpty(t *testing.T) {
+	m := NewWithStringComparator()
+	it := m.Iterator()
+	it = m.Iterator()
+	for it.Next() {
+		t.Errorf("Shouldn't iterate on empty map")
+	}
+}
+
+func TestMapIteratorPrevOnEmpty(t *testing.T) {
+	m := NewWithStringComparator()
+	it := m.Iterator()
+	it = m.Iterator()
+	for it.Prev() {
+		t.Errorf("Shouldn't iterate on empty map")
+	}
+}
+
+func TestMapIteratorNext(t *testing.T) {
 	m := NewWithStringComparator()
 	m.Put("c", 3)
 	m.Put("a", 1)
 	m.Put("b", 2)
 
 	it := m.Iterator()
+	count := 0
 	for it.Next() {
+		count++
 		key := it.Key()
 		value := it.Value()
 		switch key {
@@ -337,12 +357,52 @@ func TestMapIterator(t *testing.T) {
 		default:
 			t.Errorf("Too many")
 		}
+		if actualValue, expectedValue := value, count; actualValue != expectedValue {
+			t.Errorf("Got %v expected %v", actualValue, expectedValue)
+		}
+	}
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
 	}
+}
 
-	m.Clear()
-	it = m.Iterator()
+func TestMapIteratorPrev(t *testing.T) {
+	m := NewWithStringComparator()
+	m.Put("c", 3)
+	m.Put("a", 1)
+	m.Put("b", 2)
+
+	it := m.Iterator()
 	for it.Next() {
-		t.Errorf("Shouldn't iterate on empty map")
+	}
+	countDown := m.Size()
+	for it.Prev() {
+		countDown--
+		key := it.Key()
+		value := it.Value()
+		switch key {
+		case "a":
+			if actualValue, expectedValue := value, 1; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case "b":
+			if actualValue, expectedValue := value, 2; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case "c":
+			if actualValue, expectedValue := value, 3; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		default:
+			t.Errorf("Too many")
+		}
+		if actualValue, expectedValue := value, countDown; actualValue != expectedValue {
+			t.Errorf("Got %v expected %v", actualValue, expectedValue)
+		}
+	}
+	// one less that in Next(), thus "1"
+	if actualValue, expectedValue := countDown, 1; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
 	}
 }
 
diff --git a/sets/hashset/hashset.go b/sets/hashset/hashset.go
index 41b78c7..e3f44c7 100644
--- a/sets/hashset/hashset.go
+++ b/sets/hashset/hashset.go
@@ -24,10 +24,11 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of set backed by a hash table.
+// Package hashset implements a set backed by a hash table.
+//
 // Structure is not thread safe.
+//
 // References: http://en.wikipedia.org/wiki/Set_%28abstract_data_type%29
-
 package hashset
 
 import (
diff --git a/sets/sets.go b/sets/sets.go
index fd4fc08..b0ae988 100644
--- a/sets/sets.go
+++ b/sets/sets.go
@@ -16,6 +16,11 @@ License along with this library. See the file LICENSE included
 with this distribution for more information.
 */
 
+// Package sets provides an abstract Set interface.
+//
+// In computer science, a set is an abstract data type that can store certain values and no repeated values. It is a computer implementation of the mathematical concept of a finite set. Unlike most other collection types, rather than retrieving a specific element from a set, one typically tests a value for membership in a set.
+//
+// Reference: https://en.wikipedia.org/wiki/Set_%28abstract_data_type%29
 package sets
 
 import "github.com/emirpasic/gods/containers"
diff --git a/sets/treeset/treeset.go b/sets/treeset/treeset.go
index 7b09138..3a0de08 100644
--- a/sets/treeset/treeset.go
+++ b/sets/treeset/treeset.go
@@ -16,10 +16,11 @@ License along with this library. See the file LICENSE included
 with this distribution for more information.
 */
 
-// Implementation of an ordered set backed by a red-black tree.
+// Package treeset implements a tree backed by a red-black tree.
+//
 // Structure is not thread safe.
-// References: http://en.wikipedia.org/wiki/Set_%28abstract_data_type%29
-
+//
+// Reference: http://en.wikipedia.org/wiki/Set_%28abstract_data_type%29
 package treeset
 
 import (
@@ -34,7 +35,7 @@ import (
 func assertInterfaceImplementation() {
 	var _ sets.Set = (*Set)(nil)
 	var _ containers.EnumerableWithIndex = (*Set)(nil)
-	var _ containers.IteratorWithIndex = (*Iterator)(nil)
+	var _ containers.ReverseIteratorWithIndex = (*Iterator)(nil)
 }
 
 // Set holds elements in a red-black tree
@@ -109,21 +110,34 @@ func (set *Set) Values() []interface{} {
 type Iterator struct {
 	index    int
 	iterator rbt.Iterator
+	tree     *rbt.Tree
 }
 
 // Iterator holding the iterator's state
 func (set *Set) Iterator() Iterator {
-	return Iterator{index: -1, iterator: set.tree.Iterator()}
+	return Iterator{index: -1, iterator: set.tree.Iterator(), tree: set.tree}
 }
 
 // Next moves the iterator to the next element and returns true if there was a next element in the container.
 // If Next() returns true, then next element's index and value can be retrieved by Index() and Value().
 // Modifies the state of the iterator.
 func (iterator *Iterator) Next() bool {
-	iterator.index++
+	if iterator.index < iterator.tree.Size() {
+		iterator.index++
+	}
 	return iterator.iterator.Next()
 }
 
+// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+// If Prev() returns true, then previous element's index and value can be retrieved by Index() and Value().
+// Modifies the state of the iterator.
+func (iterator *Iterator) Prev() bool {
+	if iterator.index >= 0 {
+		iterator.index--
+	}
+	return iterator.iterator.Prev()
+}
+
 // Value returns the current element's value.
 // Does not modify the state of the iterator.
 func (iterator *Iterator) Value() interface{} {
diff --git a/sets/treeset/treeset_test.go b/sets/treeset/treeset_test.go
index 14bb4de..ccdb16c 100644
--- a/sets/treeset/treeset_test.go
+++ b/sets/treeset/treeset_test.go
@@ -192,12 +192,29 @@ func TestSetChaining(t *testing.T) {
 	set.Add("c", "a", "b")
 }
 
-func TestSetIterator(t *testing.T) {
+func TestSetIteratorNextOnEmpty(t *testing.T) {
 	set := NewWithStringComparator()
-	set.Add("c", "a", "b")
+	it := set.Iterator()
+	for it.Next() {
+		t.Errorf("Shouldn't iterate on empty set")
+	}
+}
+
+func TestSetIteratorPrevOnEmpty(t *testing.T) {
+	set := NewWithStringComparator()
+	it := set.Iterator()
+	for it.Prev() {
+		t.Errorf("Shouldn't iterate on empty set")
+	}
+}
 
+func TestSetIteratorNext(t *testing.T) {
+	set := NewWithStringComparator()
+	set.Add("c", "a", "b")
 	it := set.Iterator()
+	count := 0
 	for it.Next() {
+		count++
 		index := it.Index()
 		value := it.Value()
 		switch index {
@@ -216,12 +233,48 @@ func TestSetIterator(t *testing.T) {
 		default:
 			t.Errorf("Too many")
 		}
+		if actualValue, expectedValue := index, count-1; actualValue != expectedValue {
+			t.Errorf("Got %v expected %v", actualValue, expectedValue)
+		}
+	}
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
 	}
+}
 
-	set.Clear()
-	it = set.Iterator()
+func TestSetIteratorPrev(t *testing.T) {
+	set := NewWithStringComparator()
+	set.Add("c", "a", "b")
+	it := set.Iterator()
+	for it.Prev() {
+	}
+	count := 0
 	for it.Next() {
-		t.Errorf("Shouldn't iterate on empty set")
+		count++
+		index := it.Index()
+		value := it.Value()
+		switch index {
+		case 0:
+			if actualValue, expectedValue := value, "a"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 1:
+			if actualValue, expectedValue := value, "b"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 2:
+			if actualValue, expectedValue := value, "c"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		default:
+			t.Errorf("Too many")
+		}
+		if actualValue, expectedValue := index, count-1; actualValue != expectedValue {
+			t.Errorf("Got %v expected %v", actualValue, expectedValue)
+		}
+	}
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
 	}
 }
 
diff --git a/stacks/arraystack/arraystack.go b/stacks/arraystack/arraystack.go
index 1e69adc..ece76a9 100644
--- a/stacks/arraystack/arraystack.go
+++ b/stacks/arraystack/arraystack.go
@@ -24,10 +24,11 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of stack backed by ArrayList.
+// Package arraystack implements a stack backed by array list.
+//
 // Structure is not thread safe.
-// References: http://en.wikipedia.org/wiki/Stack_%28abstract_data_type%29
-
+//
+// Reference: https://en.wikipedia.org/wiki/Stack_%28abstract_data_type%29#Array
 package arraystack
 
 import (
@@ -40,7 +41,7 @@ import (
 
 func assertInterfaceImplementation() {
 	var _ stacks.Stack = (*Stack)(nil)
-	var _ containers.IteratorWithIndex = (*Iterator)(nil)
+	var _ containers.ReverseIteratorWithIndex = (*Iterator)(nil)
 }
 
 // Stack holds elements in an array-list
@@ -112,7 +113,19 @@ func (stack *Stack) Iterator() Iterator {
 // If Next() returns true, then next element's index and value can be retrieved by Index() and Value().
 // Modifies the state of the iterator.
 func (iterator *Iterator) Next() bool {
-	iterator.index++
+	if iterator.index < iterator.stack.Size() {
+		iterator.index++
+	}
+	return iterator.stack.withinRange(iterator.index)
+}
+
+// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+// If Prev() returns true, then previous element's index and value can be retrieved by Index() and Value().
+// Modifies the state of the iterator.
+func (iterator *Iterator) Prev() bool {
+	if iterator.index >= 0 {
+		iterator.index--
+	}
 	return iterator.stack.withinRange(iterator.index)
 }
 
diff --git a/stacks/arraystack/arraystack_test.go b/stacks/arraystack/arraystack_test.go
index 42d7d6a..fad4ac0 100644
--- a/stacks/arraystack/arraystack_test.go
+++ b/stacks/arraystack/arraystack_test.go
@@ -92,15 +92,24 @@ func TestStackPop(t *testing.T) {
 	}
 }
 
-func TestStackIterator(t *testing.T) {
+func TestStackIteratorOnEmpty(t *testing.T) {
+	stack := New()
+	it := stack.Iterator()
+	for it.Next() {
+		t.Errorf("Shouldn't iterate on empty stack")
+	}
+}
+
+func TestStackIteratorNext(t *testing.T) {
 	stack := New()
 	stack.Push("a")
 	stack.Push("b")
 	stack.Push("c")
 
-	// Iterator
 	it := stack.Iterator()
+	count := 0
 	for it.Next() {
+		count++
 		index := it.Index()
 		value := it.Value()
 		switch index {
@@ -119,11 +128,51 @@ func TestStackIterator(t *testing.T) {
 		default:
 			t.Errorf("Too many")
 		}
+		if actualValue, expectedValue := index, count-1; actualValue != expectedValue {
+			t.Errorf("Got %v expected %v", actualValue, expectedValue)
+		}
 	}
-	stack.Clear()
-	it = stack.Iterator()
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
+	}
+}
+
+func TestStackIteratorPrev(t *testing.T) {
+	stack := New()
+	stack.Push("a")
+	stack.Push("b")
+	stack.Push("c")
+
+	it := stack.Iterator()
 	for it.Next() {
-		t.Errorf("Shouldn't iterate on empty stack")
+	}
+	count := 0
+	for it.Prev() {
+		count++
+		index := it.Index()
+		value := it.Value()
+		switch index {
+		case 0:
+			if actualValue, expectedValue := value, "c"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 1:
+			if actualValue, expectedValue := value, "b"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 2:
+			if actualValue, expectedValue := value, "a"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		default:
+			t.Errorf("Too many")
+		}
+		if actualValue, expectedValue := index, 3-count; actualValue != expectedValue {
+			t.Errorf("Got %v expected %v", actualValue, expectedValue)
+		}
+	}
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
 	}
 }
 
diff --git a/stacks/linkedliststack/linkedliststack.go b/stacks/linkedliststack/linkedliststack.go
index 843e159..75f1948 100644
--- a/stacks/linkedliststack/linkedliststack.go
+++ b/stacks/linkedliststack/linkedliststack.go
@@ -24,11 +24,11 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of stack backed by our singly linked list.
-// Used by red-black tree during in-order traversal.
+// Package linkedliststack implements a stack backed by a singly-linked list.
+//
 // Structure is not thread safe.
-// References: http://en.wikipedia.org/wiki/Stack_%28abstract_data_type%29
-
+//
+// Reference:https://en.wikipedia.org/wiki/Stack_%28abstract_data_type%29#Linked_list
 package linkedliststack
 
 import (
@@ -108,7 +108,9 @@ func (stack *Stack) Iterator() Iterator {
 // If Next() returns true, then next element's index and value can be retrieved by Index() and Value().
 // Modifies the state of the iterator.
 func (iterator *Iterator) Next() bool {
-	iterator.index++
+	if iterator.index < iterator.stack.Size() {
+		iterator.index++
+	}
 	return iterator.stack.withinRange(iterator.index)
 }
 
diff --git a/stacks/linkedliststack/linkedliststack_test.go b/stacks/linkedliststack/linkedliststack_test.go
index c9b097a..4fb0002 100644
--- a/stacks/linkedliststack/linkedliststack_test.go
+++ b/stacks/linkedliststack/linkedliststack_test.go
@@ -100,7 +100,9 @@ func TestStackIterator(t *testing.T) {
 
 	// Iterator
 	it := stack.Iterator()
+	count := 0
 	for it.Next() {
+		count++
 		index := it.Index()
 		value := it.Value()
 		switch index {
@@ -119,7 +121,14 @@ func TestStackIterator(t *testing.T) {
 		default:
 			t.Errorf("Too many")
 		}
+		if actualValue, expectedValue := index, count-1; actualValue != expectedValue {
+			t.Errorf("Got %v expected %v", actualValue, expectedValue)
+		}
+	}
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
 	}
+
 	stack.Clear()
 	it = stack.Iterator()
 	for it.Next() {
diff --git a/stacks/stacks.go b/stacks/stacks.go
index ceaf6f1..a580d36 100644
--- a/stacks/stacks.go
+++ b/stacks/stacks.go
@@ -24,6 +24,11 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
+// Package stacks provides an abstract Stack interface.
+//
+// In computer science, a stack is an abstract data type that serves as a collection of elements, with two principal operations: push, which adds an element to the collection, and pop, which removes the most recently added element that was not yet removed. The order in which elements come off a stack gives rise to its alternative name, LIFO (for last in, first out). Additionally, a peek operation may give access to the top without modifying the stack.
+//
+// Reference: https://en.wikipedia.org/wiki/Stack_%28abstract_data_type%29
 package stacks
 
 import "github.com/emirpasic/gods/containers"
diff --git a/trees/binaryheap/binaryheap.go b/trees/binaryheap/binaryheap.go
index 8a97d5a..0e5942f 100644
--- a/trees/binaryheap/binaryheap.go
+++ b/trees/binaryheap/binaryheap.go
@@ -24,11 +24,13 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of binary heap backed by ArrayList.
+// Package binaryheap implements a binary heap backed by array list.
+//
 // Comparator defines this heap as either min or max heap.
+//
 // Structure is not thread safe.
+//
 // References: http://en.wikipedia.org/wiki/Binary_heap
-
 package binaryheap
 
 import (
@@ -42,7 +44,7 @@ import (
 
 func assertInterfaceImplementation() {
 	var _ trees.Tree = (*Heap)(nil)
-	var _ containers.IteratorWithIndex = (*Iterator)(nil)
+	var _ containers.ReverseIteratorWithIndex = (*Iterator)(nil)
 }
 
 // Heap holds elements in an array-list
@@ -127,7 +129,19 @@ func (heap *Heap) Iterator() Iterator {
 // If Next() returns true, then next element's index and value can be retrieved by Index() and Value().
 // Modifies the state of the iterator.
 func (iterator *Iterator) Next() bool {
-	iterator.index++
+	if iterator.index < iterator.heap.Size() {
+		iterator.index++
+	}
+	return iterator.heap.withinRange(iterator.index)
+}
+
+// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+// If Prev() returns true, then previous element's index and value can be retrieved by Index() and Value().
+// Modifies the state of the iterator.
+func (iterator *Iterator) Prev() bool {
+	if iterator.index >= 0 {
+		iterator.index--
+	}
 	return iterator.heap.withinRange(iterator.index)
 }
 
diff --git a/trees/binaryheap/binaryheap_test.go b/trees/binaryheap/binaryheap_test.go
index 7699634..d72375a 100644
--- a/trees/binaryheap/binaryheap_test.go
+++ b/trees/binaryheap/binaryheap_test.go
@@ -107,19 +107,24 @@ func TestBinaryHeapRandom(t *testing.T) {
 	}
 }
 
-func TestBinaryHeapIterator(t *testing.T) {
+func TestBinaryHeapIteratorOnEmpty(t *testing.T) {
 	heap := NewWithIntComparator()
-
-	if actualValue := heap.Empty(); actualValue != true {
-		t.Errorf("Got %v expected %v", actualValue, true)
+	it := heap.Iterator()
+	for it.Next() {
+		t.Errorf("Shouldn't iterate on empty heap")
 	}
+}
 
+func TestBinaryHeapIteratorNext(t *testing.T) {
+	heap := NewWithIntComparator()
 	heap.Push(3) // [3]
 	heap.Push(2) // [2,3]
 	heap.Push(1) // [1,3,2](2 swapped with 1, hence last)
 
 	it := heap.Iterator()
+	count := 0
 	for it.Next() {
+		count++
 		index := it.Index()
 		value := it.Value()
 		switch index {
@@ -138,12 +143,51 @@ func TestBinaryHeapIterator(t *testing.T) {
 		default:
 			t.Errorf("Too many")
 		}
+		if actualValue, expectedValue := index, count-1; actualValue != expectedValue {
+			t.Errorf("Got %v expected %v", actualValue, expectedValue)
+		}
 	}
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
+	}
+}
 
-	heap.Clear()
-	it = heap.Iterator()
+func TestBinaryHeapIteratorPrev(t *testing.T) {
+	heap := NewWithIntComparator()
+	heap.Push(3) // [3]
+	heap.Push(2) // [2,3]
+	heap.Push(1) // [1,3,2](2 swapped with 1, hence last)
+
+	it := heap.Iterator()
 	for it.Next() {
-		t.Errorf("Shouldn't iterate on empty stack")
+	}
+	count := 0
+	for it.Prev() {
+		count++
+		index := it.Index()
+		value := it.Value()
+		switch index {
+		case 0:
+			if actualValue, expectedValue := value, 1; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 1:
+			if actualValue, expectedValue := value, 3; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 2:
+			if actualValue, expectedValue := value, 2; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		default:
+			t.Errorf("Too many")
+		}
+		if actualValue, expectedValue := index, 3-count; actualValue != expectedValue {
+			t.Errorf("Got %v expected %v", actualValue, expectedValue)
+		}
+	}
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
 	}
 }
 
diff --git a/trees/redblacktree/redblacktree.go b/trees/redblacktree/redblacktree.go
index abcdffd..cf68bab 100644
--- a/trees/redblacktree/redblacktree.go
+++ b/trees/redblacktree/redblacktree.go
@@ -24,11 +24,13 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of Red-black tree.
+// Package redblacktree implements a red-black tree.
+//
 // Used by TreeSet and TreeMap.
+//
 // Structure is not thread safe.
+//
 // References: http://en.wikipedia.org/wiki/Red%E2%80%93black_tree
-
 package redblacktree
 
 import (
@@ -41,7 +43,7 @@ import (
 
 func assertInterfaceImplementation() {
 	var _ trees.Tree = (*Tree)(nil)
-	var _ containers.IteratorWithKey = (*Iterator)(nil)
+	var _ containers.ReverseIteratorWithKey = (*Iterator)(nil)
 }
 
 type color bool
@@ -279,37 +281,65 @@ func (tree *Tree) Clear() {
 // Iterator holding the iterator's state
 type Iterator struct {
 	tree *Tree
-	left *Node
+	node *Node
 }
 
 // Iterator returns a stateful iterator whose elements are key/value pairs.
 func (tree *Tree) Iterator() Iterator {
-	return Iterator{tree: tree, left: nil}
+	return Iterator{tree: tree, node: nil}
 }
 
 // Next moves the iterator to the next element and returns true if there was a next element in the container.
 // If Next() returns true, then next element's key and value can be retrieved by Key() and Value().
 // Modifies the state of the iterator.
 func (iterator *Iterator) Next() bool {
-	if iterator.left == nil {
-		iterator.left = iterator.tree.Left()
-		return iterator.left != nil
-	}
-	if iterator.left.Right != nil {
-		iterator.left = iterator.left.Right
-		for iterator.left.Left != nil {
-			iterator.left = iterator.left.Left
+	if iterator.node == nil {
+		iterator.node = iterator.tree.Left()
+		return iterator.node != nil
+	}
+	if iterator.node.Right != nil {
+		iterator.node = iterator.node.Right
+		for iterator.node.Left != nil {
+			iterator.node = iterator.node.Left
+		}
+		return true
+	}
+	if iterator.node.Parent != nil {
+		node := iterator.node
+		for iterator.node.Parent != nil {
+			iterator.node = iterator.node.Parent
+			if iterator.tree.Comparator(node.Key, iterator.node.Key) <= 0 {
+				return true
+			}
+		}
+		iterator.node = node // fix: if parent didn't satisfy the comparator criteria
+	}
+	return false
+}
+
+// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+// If Prev() returns true, then previous element's index and value can be retrieved by Key() and Value().
+// Modifies the state of the iterator.
+func (iterator *Iterator) Prev() bool {
+	if iterator.node == nil {
+		return false
+	}
+	if iterator.node.Left != nil {
+		iterator.node = iterator.node.Left
+		for iterator.node.Right != nil {
+			iterator.node = iterator.node.Right
 		}
 		return true
 	}
-	if iterator.left.Parent != nil {
-		key := iterator.left.Key
-		for iterator.left.Parent != nil {
-			iterator.left = iterator.left.Parent
-			if iterator.tree.Comparator(key, iterator.left.Key) <= 0 {
+	if iterator.node.Parent != nil {
+		node := iterator.node
+		for iterator.node.Parent != nil {
+			iterator.node = iterator.node.Parent
+			if iterator.tree.Comparator(node.Key, iterator.node.Key) >= 0 {
 				return true
 			}
 		}
+		iterator.node = node // fix: if parent didn't satisfy the comparator criteria
 	}
 	return false
 }
@@ -317,13 +347,13 @@ func (iterator *Iterator) Next() bool {
 // Value returns the current element's value.
 // Does not modify the state of the iterator.
 func (iterator *Iterator) Value() interface{} {
-	return iterator.left.Value
+	return iterator.node.Value
 }
 
 // Key returns the current element's key.
 // Does not modify the state of the iterator.
 func (iterator *Iterator) Key() interface{} {
-	return iterator.left.Key
+	return iterator.node.Key
 }
 
 // String returns a string representation of container
diff --git a/trees/redblacktree/redblacktree_test.go b/trees/redblacktree/redblacktree_test.go
index 96fa4e1..3f5987a 100644
--- a/trees/redblacktree/redblacktree_test.go
+++ b/trees/redblacktree/redblacktree_test.go
@@ -206,7 +206,23 @@ func TestRedBlackTreeCeilingAndFloor(t *testing.T) {
 	}
 }
 
-func TestRedBlackTreeIterator1(t *testing.T) {
+func TestRedBlackTreeIteratorNextOnEmpty(t *testing.T) {
+	tree := NewWithIntComparator()
+	it := tree.Iterator()
+	for it.Next() {
+		t.Errorf("Shouldn't iterate on empty tree")
+	}
+}
+
+func TestRedBlackTreeIteratorPrevOnEmpty(t *testing.T) {
+	tree := NewWithIntComparator()
+	it := tree.Iterator()
+	for it.Prev() {
+		t.Errorf("Shouldn't iterate on empty tree")
+	}
+}
+
+func TestRedBlackTreeIterator1Next(t *testing.T) {
 	tree := NewWithIntComparator()
 	tree.Put(5, "e")
 	tree.Put(6, "f")
@@ -216,29 +232,76 @@ func TestRedBlackTreeIterator1(t *testing.T) {
 	tree.Put(1, "x")
 	tree.Put(2, "b")
 	tree.Put(1, "a") //overwrite
-
+	// │   ┌── 7
+	// └── 6
+	//     │   ┌── 5
+	//     └── 4
+	//         │   ┌── 3
+	//         └── 2
+	//             └── 1
 	it := tree.Iterator()
 	count := 0
 	for it.Next() {
 		count++
-		index := it.Key()
-		switch index {
+		key := it.Key()
+		switch key {
 		case count:
-			if actualValue, expectedValue := index, count; actualValue != expectedValue {
+			if actualValue, expectedValue := key, count; actualValue != expectedValue {
 				t.Errorf("Got %v expected %v", actualValue, expectedValue)
 			}
 		default:
-			if actualValue, expectedValue := index, count; actualValue != expectedValue {
+			if actualValue, expectedValue := key, count; actualValue != expectedValue {
 				t.Errorf("Got %v expected %v", actualValue, expectedValue)
 			}
 		}
 	}
-	if actualValue, expectedValue := count, 7; actualValue != expectedValue {
+	if actualValue, expectedValue := count, tree.Size(); actualValue != expectedValue {
 		t.Errorf("Size different. Got %v expected %v", actualValue, expectedValue)
 	}
 }
 
-func TestRedBlackTreeIterator2(t *testing.T) {
+func TestRedBlackTreeIterator1Prev(t *testing.T) {
+	tree := NewWithIntComparator()
+	tree.Put(5, "e")
+	tree.Put(6, "f")
+	tree.Put(7, "g")
+	tree.Put(3, "c")
+	tree.Put(4, "d")
+	tree.Put(1, "x")
+	tree.Put(2, "b")
+	tree.Put(1, "a") //overwrite
+	// │   ┌── 7
+	// └── 6
+	//     │   ┌── 5
+	//     └── 4
+	//         │   ┌── 3
+	//         └── 2
+	//             └── 1
+	it := tree.Iterator()
+	for it.Next() {
+	}
+	countDown := tree.size
+	for it.Prev() {
+		countDown--
+		key := it.Key()
+		switch key {
+		case countDown:
+			if actualValue, expectedValue := key, countDown; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		default:
+			if actualValue, expectedValue := key, countDown; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		}
+	}
+	// one less that in Next(), thus "1"
+	if actualValue, expectedValue := countDown, 1; actualValue != expectedValue {
+		t.Errorf("Size different. Got %v expected %v", actualValue, expectedValue)
+	}
+}
+
+func TestRedBlackTreeIterator2Next(t *testing.T) {
 	tree := NewWithIntComparator()
 	tree.Put(3, "c")
 	tree.Put(1, "a")
@@ -247,50 +310,142 @@ func TestRedBlackTreeIterator2(t *testing.T) {
 	count := 0
 	for it.Next() {
 		count++
-		index := it.Key()
-		switch index {
+		key := it.Key()
+		switch key {
 		case count:
-			if actualValue, expectedValue := index, count; actualValue != expectedValue {
+			if actualValue, expectedValue := key, count; actualValue != expectedValue {
 				t.Errorf("Got %v expected %v", actualValue, expectedValue)
 			}
 		default:
-			if actualValue, expectedValue := index, count; actualValue != expectedValue {
+			if actualValue, expectedValue := key, count; actualValue != expectedValue {
 				t.Errorf("Got %v expected %v", actualValue, expectedValue)
 			}
 		}
 	}
-	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+	if actualValue, expectedValue := count, tree.Size(); actualValue != expectedValue {
 		t.Errorf("Size different. Got %v expected %v", actualValue, expectedValue)
 	}
 }
 
-func TestRedBlackTreeIterator3(t *testing.T) {
+func TestRedBlackTreeIterator2Prev(t *testing.T) {
 	tree := NewWithIntComparator()
+	tree.Put(3, "c")
+	tree.Put(1, "a")
+	tree.Put(2, "b")
+	it := tree.Iterator()
+	for it.Next() {
+	}
+	countDown := tree.size
+	for it.Prev() {
+		countDown--
+		key := it.Key()
+		switch key {
+		case countDown:
+			if actualValue, expectedValue := key, countDown; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		default:
+			if actualValue, expectedValue := key, countDown; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		}
+	}
+	// one less that in Next(), thus "1"
+	if actualValue, expectedValue := countDown, 1; actualValue != expectedValue {
+		t.Errorf("Size different. Got %v expected %v", actualValue, expectedValue)
+	}
+}
 
+func TestRedBlackTreeIterator3Next(t *testing.T) {
+	tree := NewWithIntComparator()
+	tree.Put(1, "a")
 	it := tree.Iterator()
+	count := 0
 	for it.Next() {
-		t.Errorf("Shouldn't iterate on empty stack")
+		count++
+		key := it.Key()
+		switch key {
+		case count:
+			if actualValue, expectedValue := key, count; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		default:
+			if actualValue, expectedValue := key, count; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		}
+	}
+	if actualValue, expectedValue := count, tree.Size(); actualValue != expectedValue {
+		t.Errorf("Size different. Got %v expected %v", actualValue, expectedValue)
 	}
+}
 
+func TestRedBlackTreeIterator3Prev(t *testing.T) {
+	tree := NewWithIntComparator()
 	tree.Put(1, "a")
+	it := tree.Iterator()
+	for it.Next() {
+	}
+	countDown := tree.size
+	for it.Prev() {
+		countDown--
+		key := it.Key()
+		switch key {
+		case countDown:
+			if actualValue, expectedValue := key, countDown; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		default:
+			if actualValue, expectedValue := key, countDown; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		}
+	}
+	// one less that in Next(), thus "1"
+	if actualValue, expectedValue := countDown, 1; actualValue != expectedValue {
+		t.Errorf("Size different. Got %v expected %v", actualValue, expectedValue)
+	}
+}
 
-	it = tree.Iterator()
+func TestRedBlackTreeIterator4Next(t *testing.T) {
+	tree := NewWithIntComparator()
+	tree.Put(13, 5)
+	tree.Put(8, 3)
+	tree.Put(17, 7)
+	tree.Put(1, 1)
+	tree.Put(11, 4)
+	tree.Put(15, 6)
+	tree.Put(25, 9)
+	tree.Put(6, 2)
+	tree.Put(22, 8)
+	tree.Put(27, 10)
+	// │           ┌── 27
+	// │       ┌── 25
+	// │       │   └── 22
+	// │   ┌── 17
+	// │   │   └── 15
+	// └── 13
+	//     │   ┌── 11
+	//     └── 8
+	//         │   ┌── 6
+	//         └── 1
+	it := tree.Iterator()
 	count := 0
 	for it.Next() {
 		count++
-		index := it.Key()
-		switch index {
+		value := it.Value()
+		switch value {
 		case count:
-			if actualValue, expectedValue := index, count; actualValue != expectedValue {
+			if actualValue, expectedValue := value, count; actualValue != expectedValue {
 				t.Errorf("Got %v expected %v", actualValue, expectedValue)
 			}
 		default:
-			if actualValue, expectedValue := index, count; actualValue != expectedValue {
+			if actualValue, expectedValue := value, count; actualValue != expectedValue {
 				t.Errorf("Got %v expected %v", actualValue, expectedValue)
 			}
 		}
 	}
-	if actualValue, expectedValue := count, 1; actualValue != expectedValue {
+	if actualValue, expectedValue := count, tree.Size(); actualValue != expectedValue {
 		t.Errorf("Size different. Got %v expected %v", actualValue, expectedValue)
 	}
 }
@@ -307,11 +462,22 @@ func TestRedBlackTreeIterator4(t *testing.T) {
 	tree.Put(6, 2)
 	tree.Put(22, 8)
 	tree.Put(27, 10)
-
+	// │           ┌── 27
+	// │       ┌── 25
+	// │       │   └── 22
+	// │   ┌── 17
+	// │   │   └── 15
+	// └── 13
+	//     │   ┌── 11
+	//     └── 8
+	//         │   ┌── 6
+	//         └── 1
 	it := tree.Iterator()
-	count := 0
+	count := tree.Size()
 	for it.Next() {
-		count++
+	}
+	for it.Prev() {
+		count--
 		value := it.Value()
 		switch value {
 		case count:
@@ -324,7 +490,8 @@ func TestRedBlackTreeIterator4(t *testing.T) {
 			}
 		}
 	}
-	if actualValue, expectedValue := count, 10; actualValue != expectedValue {
+	// one less that in Next(), thus "1"
+	if actualValue, expectedValue := count, 1; actualValue != expectedValue {
 		t.Errorf("Size different. Got %v expected %v", actualValue, expectedValue)
 	}
 }
diff --git a/trees/trees.go b/trees/trees.go
index 055c308..2b235c1 100644
--- a/trees/trees.go
+++ b/trees/trees.go
@@ -24,6 +24,11 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
+// Package trees provides an abstract Tree interface.
+//
+// In computer science, a tree is a widely used abstract data type (ADT) or data structure implementing this ADT that simulates a hierarchical tree structure, with a root value and subtrees of children with a parent node, represented as a set of linked nodes.
+//
+// Reference: https://en.wikipedia.org/wiki/Tree_%28data_structure%29
 package trees
 
 import "github.com/emirpasic/gods/containers"
diff --git a/utils/comparator.go b/utils/comparator.go
index 6ce6829..7dfd698 100644
--- a/utils/comparator.go
+++ b/utils/comparator.go
@@ -29,10 +29,10 @@ package utils
 // Comparator will make type assertion (see IntComparator for example),
 // which will panic if a or b are not of the asserted type.
 //
-// Should return:
-//   -1, if a < b
-//    0, if a == b
-//    1, if a > b
+// Should return a number:
+//    negative , if a < b
+//    zero     , if a == b
+//    positive , if a > b
 type Comparator func(a, b interface{}) int
 
 // IntComparator provides a basic comparison on ints
diff --git a/utils/sort.go b/utils/sort.go
index 3cf8c1f..154ce3d 100644
--- a/utils/sort.go
+++ b/utils/sort.go
@@ -24,14 +24,13 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Util methods for sorting a slice of values with respect to the comparator
-
 package utils
 
 import "sort"
 
-// Sort sorts values (in-place)
-// Uses Go's sort (hybrid of quicksort for large and then insertion sort for smaller slices)
+// Sort sorts values (in-place) with respect to the given comparator.
+//
+// Uses Go's sort (hybrid of quicksort for large and then insertion sort for smaller slices).
 func Sort(values []interface{}, comparator Comparator) {
 	sort.Sort(sortable{values, comparator})
 }
diff --git a/utils/utils.go b/utils/utils.go
new file mode 100644
index 0000000..077c1f0
--- /dev/null
+++ b/utils/utils.go
@@ -0,0 +1,32 @@
+/*
+Copyright (c) 2015, Emir Pasic
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*/
+
+// Package utils provides common utility functions.
+//
+// Provided functionalities:
+// - sorting
+// - comparators
+package utils