struct TreeNode {    
    int val;    
    TreeNode* left;    
    TreeNode* right;    
    TreeNode(int x): val(x), left(nullptr), right(nullptr) {}
};
void insert(TreeNode*& root, int key) {    
    if (!root) {        
        root = new TreeNode(key);        
        return;    
    }    
    _______________________________
}