题目链接/文章讲解/视频讲解：https://programmercarl.com/0101.%E5%AF%B9%E7%A7%B0%E4%BA%8C%E5%8F%89%E6%A0%91.html

1.这个题还有迭代的队列和栈的实现方式，二刷的时候我要再写一遍。

CPP:递归

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    bool compare(TreeNode *left,TreeNode *right){
        if(left == NULL && right == NULL)return true;
        else if(left == NULL && right != NULL)return false;
        else if(left != NULL && right == NULL)return false;
        else if(left->val != right->val)return false;

        bool outside = compare(left->left,right->right);
        bool inside = compare(left->right,right->left);
        bool res = outside && inside;
        return res;
    }

    bool isSymmetric(TreeNode* root) {
        if(root == NULL)return true;
        return compare(root->left,root->right);
    }
};