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> map = new HashMap<>();
    public TreeNode reConstructBinaryTree(int [] pre,int [] vin) {
        if(pre == null || pre.length == 0) {
            return null;
        } else if(pre.length == 1) {
            return new TreeNode(pre[0]);
        }
        for(int i = 0; i < pre.length; i++) {
            map.put(vin[i], i);
        }
        return buildTree(pre, vin, 0, pre.length - 1, 0, pre.length - 1);
    }
    public TreeNode buildTree(int[] pre, int[] vin, int prLe, int prRi, int viLe, int viRi) {
        if(prLe > prRi) {
            return null;
        }
        int viRoot = map.get(pre[prLe]);
        int leLen = viRoot - viLe; 
        TreeNode root = new TreeNode(pre[prLe]);
        root.left  = buildTree(pre, vin, prLe + 1, prLe + leLen, viLe, viRoot - 1);
        root.right = buildTree(pre, vin, prLe + leLen + 1, prRi, viRoot + 1, viRi);
        return root;
    }
}