# 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):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if not root: return True
        return self.isMirror(root.left, root.right)

    def isMirror(self, l, r):
        if not l and not r: return True
        if not l or not r or l.val!=r.val: return False
        return self.isMirror(l.left, r.right) and self.isMirror(l.right, r.left)