Java冒泡排序

冒泡排序通过比较相邻两个数的大小来排列顺序。

难点：

需要两层循环

需要交换相邻两个数组元素的值

下面这段代码，只有一层循环。这个循环执行完毕时只是找出了最大的数，剩下的数并没有排序。

import org.junit.Test;

import java.util.Arrays;


public class Demo {

    int[] a = {2, 0, 1, 9, 3, 6, 5, 8, 4};

    @Test
    public void bubbleSort() {
        int temp = 0;
        for (int j = 0; j < a.length - 1; j++) {//注意：j<a.length-1,如果不减去1，数组会越界
            if (a[j + 1] < a[j]) {
                temp = a[j];
                a[j] = a[j + 1];
                a[j + 1] = temp;
                System.out.println(Arrays.toString(a));
            }
        }
    }
}

 上面代码的运行结果

[0, 2, 1, 9, 3, 6, 5, 8, 4]
[0, 1, 2, 9, 3, 6, 5, 8, 4]
[0, 1, 2, 3, 9, 6, 5, 8, 4]
[0, 1, 2, 3, 6, 9, 5, 8, 4]
[0, 1, 2, 3, 6, 5, 9, 8, 4]
[0, 1, 2, 3, 6, 5, 8, 9, 4]
[0, 1, 2, 3, 6, 5, 8, 4, 9]

改进上面的代码，加个外层循环

import org.junit.Test;

import java.util.Arrays;


public class Demo {

    int[] a = {2, 0, 1, 9,7, 3, 6, 5, 8, 4};

    @Test
    public void bubbleSort() {
        int temp = 0;
        for (int i = 0; i < a.length - 1; i++) {
            for (int j = 0; j < a.length - i - 1; j++) {
//注意：这里是j<a.length-i-1.
//原因，第一次循环时已经把最大数9，排到最后面了，第二次循环时就应该在剩下的数里面找出8，要少一次循环。
                if (a[j + 1] < a[j]) {
                    temp = a[j];
                    a[j] = a[j + 1];
                    a[j + 1] = temp;
                    System.out.println(Arrays.toString(a));
                }
            }
        }
    }
}

运行结果

[0, 2, 1, 9, 7, 3, 6, 5, 8, 4]
[0, 1, 2, 9, 7, 3, 6, 5, 8, 4]
[0, 1, 2, 7, 9, 3, 6, 5, 8, 4]
[0, 1, 2, 7, 3, 9, 6, 5, 8, 4]
[0, 1, 2, 7, 3, 6, 9, 5, 8, 4]
[0, 1, 2, 7, 3, 6, 5, 9, 8, 4]
[0, 1, 2, 7, 3, 6, 5, 8, 9, 4]
[0, 1, 2, 7, 3, 6, 5, 8, 4, 9]
[0, 1, 2, 3, 7, 6, 5, 8, 4, 9]
[0, 1, 2, 3, 6, 7, 5, 8, 4, 9]
[0, 1, 2, 3, 6, 5, 7, 8, 4, 9]
[0, 1, 2, 3, 6, 5, 7, 4, 8, 9]
[0, 1, 2, 3, 5, 6, 7, 4, 8, 9]
[0, 1, 2, 3, 5, 6, 4, 7, 8, 9]
[0, 1, 2, 3, 5, 4, 6, 7, 8, 9]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

来看一个最复杂的排序，这是冒泡排序空间复杂度最复杂的情况

import org.junit.Test;

import java.util.Arrays;


public class Demo {

    int[] a = {9, 8, 7, 6,5, 4, 3, 2, 1, 0};

    @Test
    public void bubbleSort() {
        int temp = 0;
        for (int i = 0; i < a.length - 1; i++) {
            for (int j = 0; j < a.length - i - 1; j++) {
                if (a[j + 1] < a[j]) {
                    temp = a[j];
                    a[j] = a[j + 1];
                    a[j + 1] = temp;
                    System.out.println(Arrays.toString(a));
                }
            }
        }
    }
}

其结果

[8, 9, 7, 6, 5, 4, 3, 2, 1, 0]
[8, 7, 9, 6, 5, 4, 3, 2, 1, 0]
[8, 7, 6, 9, 5, 4, 3, 2, 1, 0]
[8, 7, 6, 5, 9, 4, 3, 2, 1, 0]
[8, 7, 6, 5, 4, 9, 3, 2, 1, 0]
[8, 7, 6, 5, 4, 3, 9, 2, 1, 0]
[8, 7, 6, 5, 4, 3, 2, 9, 1, 0]
[8, 7, 6, 5, 4, 3, 2, 1, 9, 0]
[8, 7, 6, 5, 4, 3, 2, 1, 0, 9]
[7, 8, 6, 5, 4, 3, 2, 1, 0, 9]
[7, 6, 8, 5, 4, 3, 2, 1, 0, 9]
[7, 6, 5, 8, 4, 3, 2, 1, 0, 9]
[7, 6, 5, 4, 8, 3, 2, 1, 0, 9]
[7, 6, 5, 4, 3, 8, 2, 1, 0, 9]
[7, 6, 5, 4, 3, 2, 8, 1, 0, 9]
[7, 6, 5, 4, 3, 2, 1, 8, 0, 9]
[7, 6, 5, 4, 3, 2, 1, 0, 8, 9]
[6, 7, 5, 4, 3, 2, 1, 0, 8, 9]
[6, 5, 7, 4, 3, 2, 1, 0, 8, 9]
[6, 5, 4, 7, 3, 2, 1, 0, 8, 9]
[6, 5, 4, 3, 7, 2, 1, 0, 8, 9]
[6, 5, 4, 3, 2, 7, 1, 0, 8, 9]
[6, 5, 4, 3, 2, 1, 7, 0, 8, 9]
[6, 5, 4, 3, 2, 1, 0, 7, 8, 9]
[5, 6, 4, 3, 2, 1, 0, 7, 8, 9]
[5, 4, 6, 3, 2, 1, 0, 7, 8, 9]
[5, 4, 3, 6, 2, 1, 0, 7, 8, 9]
[5, 4, 3, 2, 6, 1, 0, 7, 8, 9]
[5, 4, 3, 2, 1, 6, 0, 7, 8, 9]
[5, 4, 3, 2, 1, 0, 6, 7, 8, 9]
[4, 5, 3, 2, 1, 0, 6, 7, 8, 9]
[4, 3, 5, 2, 1, 0, 6, 7, 8, 9]
[4, 3, 2, 5, 1, 0, 6, 7, 8, 9]
[4, 3, 2, 1, 5, 0, 6, 7, 8, 9]
[4, 3, 2, 1, 0, 5, 6, 7, 8, 9]
[3, 4, 2, 1, 0, 5, 6, 7, 8, 9]
[3, 2, 4, 1, 0, 5, 6, 7, 8, 9]
[3, 2, 1, 4, 0, 5, 6, 7, 8, 9]
[3, 2, 1, 0, 4, 5, 6, 7, 8, 9]
[2, 3, 1, 0, 4, 5, 6, 7, 8, 9]
[2, 1, 3, 0, 4, 5, 6, 7, 8, 9]
[2, 1, 0, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 0, 3, 4, 5, 6, 7, 8, 9]
[1, 0, 2, 3, 4, 5, 6, 7, 8, 9]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]