Binary Tree Postorder Traversal in Python

Suppose we have a binary tree. We have to find the post order traversal of this tree using the 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

Example

Let us see the following implementation to get a better understanding −

 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))

Input

[-10,9,10,None,None,15,7]

Output

[9, 15, 7, 10, -10]