题目很直接，很简单啊 
    TreeNode* Mirror(TreeNode* pRoot) {
        // write code here
        if(pRoot == NULL){
            return NULL;
        }
        
        TreeNode* left = Mirror(pRoot->left);
        TreeNode* right = Mirror(pRoot->right);
        pRoot->left = right;
        pRoot->right = left;
        return pRoot;
    }