树、递归————对称二叉树

 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 ismirror(root,root);
14     }
15     bool ismirror(TreeNode* t1,TreeNode* t2){
16         if(t1==NULL && t2==NULL) return true;//最基本单元情况两个指针都是空
17         if(t1==NULL || t2==NULL) return false;//只有一个空，那就返回false，不镜像了
18         if(t1->val != t2->val) return false;
19         return ismirror(t1->left,t2->right)&&ismirror(t1->right,t2->left);
20     }
21 };