题解 | #重建二叉树#

import java.util.*;
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private Map<Integer,Integer> indexs = new HashMap<>();
    public TreeNode reConstructBinaryTree(int [] pre,int [] vin) {
        for(int i = 0; i < vin.length; i++){
            indexs.put(vin[i],i);
        }
        return helper(pre,vin,0,vin.length-1);
    }
    int i = 0;
    // [left,right];
    private TreeNode helper(int[] pre,int[] vin,int left,int right){
        if(left > right) return null;
        TreeNode root = new TreeNode(pre[i]);
        int index_mid = indexs.get(pre[i]);
        i++;
        root.left = helper(pre,vin,left,index_mid-1);
        root.right = helper(pre,vin,index_mid+1,right);
        return root;
    }
    
}