一个二叉树非递归遍历小问题

void inordertraversenocur(BST T)
{
    stack<struct BSTnode *> S;
    BST p = T;
    BST q = (BST)malloc(sizeof(node));
    while (p || !S.empty());
    {
        if (p)
        {
            S.push(p);
            p = p->lchild;
        }
        else
        {
            q = S.top();
            S.pop();
            printf("%d ",p->num);
            p = q->rchild;
        }
    }
}