leetcode.com/python/101_Symmetric_Tree.py

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


# Iterative solution, Wrong answer but not sure why
class Solution(object):

    def isSymmetric(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if root is None:
            return True
        stack = [(root.left, root.right)]
        while stack:
            left, right = stack.pop()
            if left is None and right is None:
                return True
            if left is None or right is None:
                return False
            if left.val != right.val:
                return False
            stack.append((left.left, right.right))
            stack.append((left.right, right.left))
        return True


# Recursive solution
class Solution(object):
    def isSymmetric(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if root is None:
            return True
        return self.isMirrorByDFS(root.left, root.right)

    def isMirrorByDFS(self, node1, node2):
        if node1 is None and node2 is None:
            return True
        if node1 is None or node2 is None:
            return False
        return node1.val == node2.val and self.isMirrorByDFS(node1.left, node2.right) and self.isMirrorByDFS(node2.left, node1.right)