int TreeDepth(struct TreeNode* pRoot ) {
    if(pRoot==NULL){
        return 0;
    }
    int depth;
    int left_depth=TreeDepth(pRoot->left);
    int right_depth=TreeDepth(pRoot->right);
    depth=left_depth>right_depth?left_depth+1:right_depth+1;
    return depth;
}