101. 对称二叉树

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

class Solution:
    def isSymmetric(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        def f(p,q):
            if p == None:
                return q == None
            if q == None:
                return p == None
            if p.val == q.val:
                return f(p.left,q.right) and f(p.right,q.left)
            if p.val != q.val:
                return False
        if root == None:
            return True
        return f(root.left,root.right)