Add iteration version (#322)

traversals/binary_tree_traversals.py

@@ -46,7 +46,6 @@ def build_tree():
         node_found.right = right_node
         q.put(right_node)
 
-
 def pre_order(node):
     if not isinstance(node, TreeNode) or not node:
         return
@@ -54,7 +53,6 @@ def pre_order(node):
     pre_order(node.left)
     pre_order(node.right)
 
-
 def in_order(node):
     if not isinstance(node, TreeNode) or not node:
         return
@@ -84,6 +82,50 @@ def level_order(node):
         if node_dequeued.right:
             q.put(node_dequeued.right)
 
+#iteration version
+def pre_order_iter(node):
+    if not isinstance(node, TreeNode) or not node:
+        return
+    stack = []
+    n = node
+    while n or stack:
+        while n:    #start from root node, find its left child
+            print(n.data, end=" ")
+            stack.append(n)
+            n = n.left
+        #end of while means current node doesn't have left child
+        n = stack.pop()
+        #start to traverse its right child
+        n = n.right
+
+def in_order_iter(node):
+    if not isinstance(node, TreeNode) or not node:
+        return
+    stack = []
+    n = node
+    while n or stack:
+        while n:
+            stack.append(n)
+            n = n.left
+        n = stack.pop()
+        print(n.data, end=" ")
+        n = n.right
+
+def post_order_iter(node):
+    if not isinstance(node, TreeNode) or not node:
+        return
+    stack1, stack2 = [], []
+    n = node
+    stack1.append(n)
+    while stack1:   #to find the reversed order of post order, store it in stack2
+        n = stack1.pop()
+        if n.left:
+            stack1.append(n.left)
+        if n.right:
+            stack1.append(n.right)
+        stack2.append(n)
+    while stack2:   #pop up from stack2 will be the post order
+        print(stack2.pop().data, end=" ")
 
 if __name__ == '__main__':
     print("\n********* Binary Tree Traversals ************\n")
@@ -104,3 +146,15 @@ if __name__ == '__main__':
     print("\n********* Level Order Traversal ************")
     level_order(node)
     print("\n******************************************\n")
+
+    print("\n********* Pre Order Traversal - Iteration Version ************")
+    pre_order_iter(node)
+    print("\n******************************************\n")
+
+    print("\n********* In Order Traversal - Iteration Version ************")
+    in_order_iter(node)
+    print("\n******************************************\n")
+
+    print("\n********* Post Order Traversal - Iteration Version ************")
+    post_order_iter(node)
+    print("\n******************************************\n")