Binary Tree Postorder Traversal

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param {TreeNode} root
    # @return {integer[]}
    ''' recursive
    def postorderTraversal(self, root):
        result=[]
        if root:
            result.append(root.val)
            # right first
            if root.right:
                result=self.postorderTraversal(root.right)+result
            if root.left:
                result=self.postorderTraversal(root.left)+result
        return result
    '''
    # iterative
    def postorderTraversal(self,root):
        if not root:
            return []
        waitlist,result=[(root,False)],[]
        while waitlist:
            p,visited=waitlist.pop()
            if p:
                if not visited:
                    waitlist.append((p,True))
                    waitlist.append((p.right,False))
                    waitlist.append((p.left,False))
                else:
                    result.append(p.val)
        return result