int preorder_tree_walk(BinTreeNode * root){    if(root == NULL){        return -1;    }    stack<BinTreeNode *> s;    BinTreeNode * p = root;    while(!s.empty() || p != NULL)    {        while(p != NULL){            cout << p->key<< endl;            s.push(p);            p = p->lchild;        }        p = s.top();        s.pop();        p = p->rchild;    }    return 0;}