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 || vin == null) {
            return null;
        }
        for (int i = 0; i < vin.length; i++) {
            map.put(vin[i], i);
        }
        return reConstructBinaryTree(pre, 0, pre.length - 1, vin, 0, vin.length - 1);
    }


    private TreeNode reConstructBinaryTree(int[] pre, int pi, int pj, int[] in, int ni, int nj) {
        if (pi > pj) {
            return null;
        }
        TreeNode root = new TreeNode(pre[pi]);
        int index = map.get(pre[pi]);
        root.left = reConstructBinaryTree(pre, pi + 1, index + pi - ni, in, ni, index - 1);
        root.right = reConstructBinaryTree(pre, index + pi - ni + 1, pj, in, index + 1, nj);
        return root;
    }
}