Binary Tree Postorder Traversal in Python Programming

Suppose we have a binary tree. We have to find the post order traversal of this tree using iterative approach. So if the tree is like −

Then the output will be: [9,15,7,10,-10]

To solve this, we will follow these steps −

if root is null, then return empty array
create an array ret
stack := define a stack with pair [root, 0]
while stack is not empty −
- node := top of stack, then delete element from stack.
- if second value of node pair is 0, then
  - current := first value of node pair
  - insert a pair (current, 1) into stack
  - if right of current is present
    - insert a pair [right of current, 0] into stack
  - if left of current is present −
    - insert a pair [left of current, 0] into stack
- Otherwise when data of the first value node pair is not 0, then insert the data of the first value of node into res
return res

Let us see the following implementation to get better understanding −

Example

Live Demo

class TreeNode:
def __init__(self, data, left = None, right = None):
   self.data = data
   self.left = left
   self.right = right
def insert(temp,data):
   que = []
   que.append(temp)
   while (len(que)):
      temp = que[0]
      que.pop(0)
      if (not temp.left):
         if data is not None:
            temp.left = TreeNode(data)
         else:
            temp.left = TreeNode(0)
         break
      else:
         que.append(temp.left)
      if (not temp.right):
         if data is not None:
            temp.right = TreeNode(data)
         else:
            temp.right = TreeNode(0)
         break
      else:
         que.append(temp.right)
def make_tree(elements):
   Tree = TreeNode(elements[0])
   for element in elements[1:]:
      insert(Tree, element)
   return Tree
class Solution(object):
   def postorderTraversal(self, root):
      if not root:
         return []
      res = []
      stack = [[root,0]]
      while stack:
         node = stack[-1]
         stack.pop()
         if node[1]== 0 :
            current = node[0]
            stack.append([current,1])
            if current.right:
               stack.append([current.right,0])
            if current.left:
               stack.append([current.left,0])
         else:
            if node[0].data != 0:
               res.append(node[0].data)
      return res
ob = Solution()
root = make_tree([-10,9,10,None,None,15,7])
print(ob.postorderTraversal(root))