AcWing 38. 二叉树的镜像（C++）- 二叉树

题目链接：https://www.acwing.com/problem/content/37/
题目如下：

/**
 * 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:
    void mirror(TreeNode* root) {
    
    
        if(root==NULL) return ;
        dfs(root);
    }
    
    void dfs(TreeNode* root){
    
    
        if(root==NULL) return ;
        
        TreeNode* temp=root->left;
        root->left=root->right;
        root->right=temp;
        
        dfs(root->left);
        dfs(root->right);
    }
};