AcWing 39. 对称的二叉树（C++）- 二叉树

题目链接：https://www.acwing.com/problem/content/38/
题目如下：

class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(root==NULL) return true;
        
        return dfs(root->left,root->right);
    }
    
    bool dfs(TreeNode* left,TreeNode* right){
        if(left==NULL&&right==NULL) return true;
        
        if(left==NULL&&right!=NULL) return false;
        if(left!=NULL&&right==NULL) return false;
        if(left->val!=right->val) return false;
        
        return dfs(left->left,right->right)&&dfs(left->right,right->left);
    }
};