选择排序(Selection Sort)

  以前写过排序算法,但是主要写的是算法的概念和实现代码,本次主要写下算法的执行过程。

  好了,还是先回忆下概念。选择排序是指每次循环都把剩余数据中最小的(最大的)数找出来,从而在进过多轮循环之后得出有序数据。选择排序的时间复杂度为O(n*n).

  假设有一组数组,{99,67,82,43,12,26,18,55},则其选择从小到大的排序过程如下:

        {99,67,82,43,12,26,18,55}

  第一次循环:{67,99,82,43,12,26,18,55}

        {43,99,82,67,12,26,18,55}

        {12,99,82,67,43,26,18,55}

  第二次循环:{12,99,82,67,43,26,18,55}

        {12,82,99,67,43,26,18,55}

        {12,67,99,82,43,26,18,55}

        {12,43,99,82,67,26,18,55}

        {12,26,99,82,67,43,18,55}

        {12,18,99,82,67,43,26,55}

  第三次循环:{12,18,99,82,67,43,26,55}

        {12,18,82,99,67,43,26,55}

        {12,18,67,99,82,43,26,55}

        {12,18,43,99,82,67,26,55}

        {12,18,26,99,82,67,43,55}

  第四次循环:{12,18,26,99,82,67,43,55}

        {12,18,26,82,99,67,43,55}

        {12,18,26,67,99,82,43,55}

        {12,18,26,43,99,82,67,55}

  第五次循环:{12,18,26,43,99,82,67,55}

        {12,18,26,43,82,99,67,55}

        {12,18,26,43,67,99,82,55}

        {12,18,26,43,55,99,82,67}

  第六次循环:{12,18,26,43,55,99,82,67}

        {12,18,26,43,55,82,99,67}

        {12,18,26,43,55,67,99,82}

  第七次循环:{12,18,26,43,55,67,99,82}

        {12,18,26,43,55,67,82,99}

  第八次循环:{12,18,26,43,55,67,82,99}

  至此,排序完成,可以看出选择排序是不稳定排序。

  下面是选择排序的代码实现:

public class MainTest {

    public static void main(String[] args){
        int[] arr = {10,9,8,7,6,5,4,3,2,1};
        selectingSort(arr);

        for (int i : arr){
            System.out.println(i);
        }
    }


    public static void selectingSort(int[] arr){
        int arrLength = arr.length;
        for (int i=0; i<arrLength; i++){
            int minIndex = i;
            for (int j=i+1; j<arrLength; j++){
                if (arr[j]<arr[minIndex]){
                    minIndex = j;
                }
            }
            exchange(arr, i, minIndex);
        }
    }

    private static void exchange(int[] arr, int nowIndex, int minIndex){
        int num = arr[nowIndex];
        arr[nowIndex] = arr[minIndex];
        arr[minIndex] = num;

    }
}

 

posted @ 2017-12-24 22:05  aston  阅读(239)  评论(0编辑  收藏  举报