剑指Offer:4.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.HashMap;
import java.util.Map;
public class Solution {
    Map<Integer,Integer>map=new HashMap<>();
    public TreeNode reConstructBinaryTree(int [] pre,int [] in) {
        for(int i=0;i<in.length;i++)
            map.put(in[i],i);
        return reConstructBinaryTree(pre,0,pre.length-1,0);
    }
    
    public TreeNode reConstructBinaryTree(int[]pre,int preL,int preR,int inL){
       if(preL>preR)return null;
        TreeNode root=new TreeNode(pre[preL]);
        int index=map.get(root.val);
        int leftTreeSize=index-inL;
       root.left=reConstructBinaryTree(pre,preL+1,preL+leftTreeSize,inL);
        root.right=reConstructBinaryTree(pre,preL+leftTreeSize+1,preR,inL+leftTreeSize+1);
        return root;
    }
}