new_contents: # Definition for a  binary tree node
  # class TreeNode:
  #     def __init__(self, x):
  #         self.val = x
  #         self.left = None
  #         self.right = None
  
  
  class Solution:
      # @param root, a tree node
      # @return a list of integers
  
      def postorderTraversal(self, root):
          if root is None:
              return []
          res = []
          stk = [root]
          while stk:
              top = stk[-1]
              if top.left is None and top.right is None:
                  res.append(stk.pop().val)
                  continue
              if top.right:
                  stk.append(top.right)
                  top.right = None
              if top.left:
                  stk.append(top.left)
                  top.left = None
          return res
  
subject: add solution for Binary Tree Postorder Traversal