琢磨了半天，一看题解，我人傻了

/**
 * 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) return;

        swap(root->left, root->right);  //这里交换的是指针，不是结构体

        mirror(root->left);
        mirror(root->right);
    }
};