剑指 offer 学习之重建二叉树

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。

题目链接：牛客网

解题思路

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    
    // 缓存中序遍历数组每个值对应的索引
    private Map<Integer, Integer> indexForInOrders = new HashMap<>();

    public TreeNode reConstructBinaryTree(int[] pre, int[] in) {
        for (int i = 0; i < in.length; i++) {
            indexForInOrders.put(in[i], i);
        }
        
        return reConstructBinaryTree(pre, 0, pre.length - 1, 0);
    }

    private TreeNode reConstructBinaryTree(int[] pre, int preL, int preR, int inL) {
        if (preL > preR) {
            return null;
        }
        
        TreeNode root = new TreeNode(pre[preL]);
        int inIndex = indexForInOrders.get(root.val);
        int leftTreeSize = inIndex - inL;
        root.left = reConstructBinaryTree(pre, preL + 1, preL + leftTreeSize, inL);
        root.right = reConstructBinaryTree(pre, preL + leftTreeSize + 1, preR, inL + leftTreeSize + 1);
        return root;
    }
}

测试结果

image.png