选择排序（排序算法--比较排序）

2024年6月3日 164次阅读

第二篇算法。

选择排序，也是非常简单的排序算法，学校课本中学过。

定义如下：首先在未排序序列中找到最小（大）元素，存放到排序序列的起始位置，然后，再从剩余未排序元素中继续寻找最小（大）元素，然后放到已排序序列的末尾。以此类推，直到所有元素均排序完毕。（摘自维基百科）

Java语言实现：

        int[] arrays = {34, 32, 22, 82, 55, 89, 50, 37, 5, 3};
        System.out.println("y: " + Arrays.toString(arrays));
        int size = arrays.length;
        for (int i = 0; i < size - 1; ++i){
            int t = i;
            for (int j = i+1; j < size ; ++j){
                if (arrays[t] > arrays[j]){
                    t = j;
                }
            }
            int tmp = arrays[t];
            arrays[t] = arrays[i];
            arrays[i] = tmp;
            System.out.println( i + ": " + Arrays.toString(arrays));
        }

输出的结果为：

y: [34, 32, 22, 82, 55, 89, 50, 37, 5, 3]
0: [3, 32, 22, 82, 55, 89, 50, 37, 5, 34]
1: [3, 5, 22, 82, 55, 89, 50, 37, 32, 34]
2: [3, 5, 22, 82, 55, 89, 50, 37, 32, 34]
3: [3, 5, 22, 32, 55, 89, 50, 37, 82, 34]
4: [3, 5, 22, 32, 34, 89, 50, 37, 82, 55]
5: [3, 5, 22, 32, 34, 37, 50, 89, 82, 55]
6: [3, 5, 22, 32, 34, 37, 50, 89, 82, 55]
7: [3, 5, 22, 32, 34, 37, 50, 55, 82, 89]
8: [3, 5, 22, 32, 34, 37, 50, 55, 82, 89]


如果想要将最大的排到最前面，需要将

if (arrays[t] > arrays[j])

改为:

if (arrays[t] < arrays[j])


修改后的输出如下：

y: [34, 32, 22, 82, 55, 89, 50, 37, 5, 3]
0: [89, 32, 22, 82, 55, 34, 50, 37, 5, 3]
1: [89, 82, 22, 32, 55, 34, 50, 37, 5, 3]
2: [89, 82, 55, 32, 22, 34, 50, 37, 5, 3]
3: [89, 82, 55, 50, 22, 34, 32, 37, 5, 3]
4: [89, 82, 55, 50, 37, 34, 32, 22, 5, 3]
5: [89, 82, 55, 50, 37, 34, 32, 22, 5, 3]
6: [89, 82, 55, 50, 37, 34, 32, 22, 5, 3]
7: [89, 82, 55, 50, 37, 34, 32, 22, 5, 3]
8: [89, 82, 55, 50, 37, 34, 32, 22, 5, 3]

发现在 4 时就已经排好顺序了。

可以对此进行优化，加入flag的判断：

        int[] arrays = {34, 32, 22, 82, 55, 89, 50, 37, 5, 3};
        System.out.println("y: " + Arrays.toString(arrays));
        int size = arrays.length;
        for (int i = 0; i < size - 1; ++i){
            int t = i;
            boolean flag = true;
            for (int j = i+1; j < size ; ++j){
                if (arrays[t] < arrays[j]){
                    t = j;
                    flag = false;
                }
            }
            int tmp = arrays[t];
            arrays[t] = arrays[i];
            arrays[i] = tmp;
            System.out.println( i + ": " + Arrays.toString(arrays));
            if (flag){
                break;
            }
        }

优化后的输出结果如下：

y: [34, 32, 22, 82, 55, 89, 50, 37, 5, 3]
0: [89, 32, 22, 82, 55, 34, 50, 37, 5, 3]
1: [89, 82, 22, 32, 55, 34, 50, 37, 5, 3]
2: [89, 82, 55, 32, 22, 34, 50, 37, 5, 3]
3: [89, 82, 55, 50, 22, 34, 32, 37, 5, 3]
4: [89, 82, 55, 50, 37, 34, 32, 22, 5, 3]
5: [89, 82, 55, 50, 37, 34, 32, 22, 5, 3]

可以看到，少几次的循环。

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