剑指Offer（四）重建二叉树

题目描述

输入某二叉树的前序遍历和中序遍历的结果，请重建出该二叉树。假设输入的前序遍历和中序遍历的结果中都不含重复的数字。例如输入前序遍历序列{1,2,4,7,3,5,6,8}和中序遍历序列{4,7,2,1,5,3,8,6}，则重建二叉树并返回。

/**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public TreeNode reConstructBinaryTree(int [] pre,int [] in) {
return build(pre, in, 0, pre.length-1, 0, in.length-1);
}

public TreeNode build(int[] pre, int[] in, int pstart, int pend, int istart, int iend){
if(pstart > pend)   return null;

int cur = pre[pstart];//二叉树的根节点
int find = istart;//游标
while(find <= iend){//在中序遍历序列中寻找根节点
if(cur == in[find]) break;
else find++;
}

int len = find-istart;//子树高度
TreeNode res = new TreeNode(cur);//创建子树根节点
res.left = build(pre, in, pstart+1, pstart+len, istart, find-1);//创建左子树
res.right = build(pre, in, pstart+len+1, pend,  find+1, iend);//创建右子树
return res;
}
}