如何去检查对称？
思路是check(n1->left,n2->right)&&check(n1->right,n2->left)

直接手算这题，该懂就懂了！

/**
 * 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 isSymmetric(TreeNode* root) {
        return check(root,root);
        
    }
    bool check(TreeNode* n1,TreeNode*n2){
        if(!n1&&!n2){
            return true;
        }
        if(n1&&n2){
            return n1->val==n2->val&&check(n1->left,n2->right)&&check(n1->right,n2->left);
        }
        return false;
    }
};