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 sections, in front of the left subtree, right subtree is behind the next steps to basically repeat the first two steps* /