Code Interview Guide for Programmers Chapter 3 Binary Tree Problems Generate Postorder Arrays from Preorder and Inorder Arrays

topic

通过先序和中序数组生成后序数组

java code

package com.lizhouwei.chapter3;

import java.util.HashMap;
import java.util.Map;

/**
 * @Description:通过先序和中序数组生成后序数组
 * @Author: lizhouwei
 * @CreateDate: 2018/4/16 21:21
 * @Modify by:
 * @ModifyDate:
 */
public class Chapter3_22 {
    public int[] getPosArray(int[] pre, int[] in) {
        if (pre == null || in == null) {
            return null;
        }
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < in.length; i++) {
            map.put(in[i], i);
        }
        int[] pos = new int[in.length];
        preAndIn(pre, 0, pre.length - 1, in, 0, in.length - 1, pos, pos.length - 1, map);
        return pos;
    }

    public int preAndIn(int[] pre, int preStart, int preEnd, int[] in, int inStart, int inEnd, int[] pos, int posEnd, Map<Integer, Integer> map) {
        if (preStart > preEnd) {
            return posEnd;
        }
        int vlaue = pre[preStart];
        pos[posEnd--] = vlaue;
        int index = map.get(vlaue);

        posEnd = preAndIn(pre, preStart + index - inStart + 1, preEnd, in, index + 1, inEnd, pos, posEnd, map);
        posEnd = preAndIn(pre, preStart + 1, preStart + index - inStart, in, inStart, index - 1, pos, posEnd, map);
        return posEnd;
    }

    //测试
    public static void main(String[] args) {
        Chapter3_22 chapter = new Chapter3_22();
        int[] pre = {5, 2, 1, 3, 4, 8, 6, 7, 9, 10};
        int[] in = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        int[] pos = chapter.getPosArray(pre, in);
        System.out.println("前序数组：pre = {5, 2, 1, 3, 4, 8, 6, 7, 9, 10}");
        System.out.println("中序数组：in = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}");
        System.out.println("后序数组：pos = {1, 4, 3, 2, 7, 6, 10, 9, 8, 5}");
        System.out.println();
        System.out.print("前序和中序组成后序:");
        for (int i : pos) {
            System.out.print(i + " ");
        }
    }
}

result