一个有4层结点的完全二叉树。按前序遍历周游给结点从1开始编号，则第21号结点的父结点是多少号？（注释：根的层数为0）For a complete binary tree with four levels, label the nodes starting from 1 according to the preorder traversal. what is the label of the parent node for node 21? (The root is on level 0)