04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。

Java实现

import java.util.HashMap;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    public TreeNode (int val) {
        this.val = val;
    }
}

public class Solution {
    private HashMap<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);
    }

    public 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, inIndex + 1);
        return root;
    }
}