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; } * } */import java.util.*;public class Solution {    public TreeNode reConstructBinaryTree(int[] pre, int[] in) {        HashMap<Integer, Integer> map = new HashMap<Integer,Integer>();        for(int i =0;i<in.length;i++){            map.put(in[i], i);        }        return reconst(pre, 0, pre.length - 1, in, 0, in.length - 1,map);    }    public TreeNode reconst(int[] pre, int prei, int prej, int[] in, int ini, int inj,HashMap<Integer, Integer> map) {        int num = 0;        TreeNode head = null;        if (prei <= prej) {            head = new TreeNode(pre[prei]);            num  = map.get(pre[prei])-ini;            head.left = reconst(pre, prei + 1, prei + num, in, ini, ini + num - 1,map);            head.right = reconst(pre, prei + num + 1, prej, in, ini + num + 1, inj,map);        } else {            return null;        }        return head;    }}