树、递归————对称二叉树

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        return ismirror(root,root);
    }
    bool ismirror(TreeNode* t1,TreeNode* t2){
        if(t1==NULL && t2==NULL) return true;//最基本单元情况两个指针都是空
        if(t1==NULL || t2==NULL) return false;//只有一个空，那就返回false，不镜像了
        if(t1->val != t2->val) return false;
        return ismirror(t1->left,t2->right)&&ismirror(t1->right,t2->left);
    }
};