1由先序序列确定根节点（即第一个字母） 2按根节点把中序序列分为两段，前面是左子树，后面的是右子树 后面的步骤就基本是前面两步的重复*/ -1 determined by the sequence of the first order root (ie the first letter) 2 sequence at the root node in the sequence to be divided into two se