《程序员代码面试指南》第八章 数组和矩阵问题 找到无序数组中最小的k 个数

Posted 2020-11-01 lizhouwei

题目

找到无序数组中最小的k 个数

java代码

package com.lizhouwei.chapter8;

/**
 * @Description: 找到无序数组中最小的k 个数
 * @Author: lizhouwei
 * @CreateDate: 2018/4/29 7:37
 * @Modify by:
 * @ModifyDate:
*/
public class Chapter8_4 {

    //使用堆排序 时间复杂度为O(logN)
    public int[] getKMin(int[] arr, int k) {
        if (k > arr.length) {
            return new int[]{-1};
        }
        int[] res = new int[k];
        for (int i = 0; i < k; i++) {
            res[i] = arr[i];
            headInsert(res, i);
        }
        for (int i = k; i < arr.length; i++) {
            if (arr[i] < res[0]) {
                res[0] = arr[i];
                heapify(arr, 0, k);
            }
        }
        return res;
    }

    //堆初始化
    public void headInsert(int[] arr, int index) {
        int parent = (index - 1) / 2;
        while (index > 0) {
            if (arr[parent] < arr[index]) {
                swap(arr, parent, index);
                index = parent;
            } else {
                break;
            }
        }
    }

    //堆排序
    public void heapify(int[] arr, int index, int heapSize) {
        int left = 2 * index + 1;
        int right = 2 * index + 2;
        int largest = index;
        while (left < heapSize) {
            if (arr[left] > arr[index]) {
                largest = left;
            } else if (right < heapSize && arr[left] > arr[right]) {
                largest = right;
            } else {
                break;
            }
            swap(arr, largest, index);
            index = largest;
            left = 2 * index + 1;
            right = 2 * index + 2;
        }

    }

    public void swap(int[] arr, int a, int b) {
        int temp = arr[a];
        arr[a] = arr[b];
        arr[b] = temp;
    }

    //测试
    public static void main(String[] args) {
        Chapter8_4 chapter = new Chapter8_4();
        int[] arr = {1, 4, 7, 2, 3, 10, 9, 6, 5, 8};
        int[] res = chapter.getKMin(arr, 5);
        System.out.print("{1, 4, 7, 2, 3, 10, 9, 6, 5, 8}中前五：");
        for (int i = 0; i < res.length; i++) {
            System.out.print(res[i]+" ");
        }
    }
}

结果