58.对称的二叉树

1.题目

请实现一个函数，用来判断一颗二叉树是不是对称的。注意，如果一个二叉树同此二叉树的镜像是同样的，定义其为对称的。

2.思路

递归的思想，首先判断头结点是否为空。然后将根节点的左右两个节点假设成两个独立的树，如果左右两个树都为空，返回True。然后看左子树的左结点和右子树的右结点、左子树的右结点和右子树的左结点是否相同，都相同返回True.

3.代码

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def isSymmetrical(self, pRoot):
        # write code here
        if pRoot is None:
            return True
        return self.isSymmetricTree(pRoot.left, pRoot.right)
    
    def isSymmetricTree(self, left, right):
        if left is None and right is None:
            return True
        if left is None or right is None or left.val != right.val:
            return False
        return self.isSymmetricTree(left.left, right.right) and self.isSymmetricTree(left.right, right.left)