101. Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

Note:
Bonus points if you could solve it both recursively and iteratively.

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

class Solution(object):
    def isSymmetric(self, root):
        if not root:
            return True
        return self.is_reverse(root.left, root.right)


    def is_reverse(self, a, b):
        # return true if `a` is a reversed tree of `b`
        if not a or not b:
            return not a and not b
        return a.val == b.val and self.is_reverse(a.left,b.right) and self.is_reverse(a.right,b.left)      

以上