/**
 * struct TreeNode {
 *  int val;
 *  struct TreeNode *left;
 *  struct TreeNode *right;
 *  TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
  public:
    TreeNode* Mirror(TreeNode* pRoot) {
        if (pRoot == nullptr)return nullptr;
        TreeNode* le = Mirror(pRoot->left), *ri = Mirror(pRoot->right);
        pRoot->left = ri, pRoot->right = le;
        return pRoot;
    }
};