LeetCode 101. 对称二叉树

题目链接：​​https://leetcode-cn.com/problems/symmetric-tree/​​

给定一个二叉树，检查它是否是镜像对称的。

1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     bool isSymmetric(TreeNode *root) {
13         return root ? isSymmetric(root->left, root->right) : true;
14     }
15     bool isSymmetric(TreeNode *left, TreeNode *right) {
16         if (!left && !right) return true; 
17         if (!left || !right) return false;
18         return left->val == right->val && isSymmetric(left->left, right->right) && isSymmetric(left->right, right->left);
19     }
20 };