快速排序方法出了什么问题？答案

【问题标题】：What goes wrong with the quick sort method?快速排序方法出了什么问题？
【发布时间】：2015-10-25 15:41:58
【问题描述】：

我有一个快速排序方法测试程序如下：

public class testtt {

    public static void main(String[] args){

        Integer[] list =new Integer[15];
        list[0] =12;
        list[1] = 28;
        list[2] = 79;
        list[3] = 19;
        list[4] = 60;
        list[5] = 22;
        list[6] = 3;
        list[7] = 50;
        list[8] = 75;
        list[9] = 60;
        list[10] = 25;
        list[11] = 97;
        list[12] = 98;
        list[13] = 12;
        list[14] = 88;

        quickSort(list,0,14);

        for(int i=0;i<15;i++){
            System.out.print(list[i]+"\t");
        }



    }
    private static <E extends Comparable<E>> void quickSort(E[] list, int first, int last) {
        if (last > first) {
            int pivotIndex;
            pivotIndex = partition3(list, first, last);
            quickSort(list, first, pivotIndex - 1);
            quickSort(list, pivotIndex + 1, last);
        }
    }

    private static <E extends Comparable<E>> int partition3(E[] list, int first, int last) {
        int pivotIndex = (first + last) / 2;
        E pivot = list[pivotIndex]; // Choose the first element as the pivot
        swap(list, last, pivotIndex);
        pivotIndex = last;
        last--;
        do {
            // Search forward from left
            while (first < last && list[first].compareTo(pivot) <= 0)
                first++;
            // Search backward from right
            while (first <= last && list[last].compareTo(pivot) > 0)
                last--;

            // Swap two elements in the list
            if (last >= first) {
                swap(list, first, last);
                first++;
                last--;
            }
        } while (last > first);

        swap(list, pivotIndex, first);

        return first;
    }
    private static <E> void swap(E[] list, int index1, int index2) {
        E tmp = list[index1];
        list[index1] = list[index2];
        list[index2] = tmp;
    }
}

运行输出结果为

3   12  19  22  25  12  28  50  60  60  75  79  88  97  98

未排序...有人能解释一下为什么这个方法出错了吗？我不需要正确版本的快速排序，我只想知道为什么这个错误...我无法在 python 导师或其他什么上进行可视化，因为运行时间超过 3 秒...

【问题讨论】：

在每一步之间显示列表的内容，看看哪里出了问题。

标签： java sorting quicksort

【解决方案1】：

您必须在 partition 方法的 do...while 循环中将 > 替换为 >= ：

private static <E extends Comparable<E>> int partition3(E[] list, int first, int last) {
        // Code 
        do {
            // Code 
        } while (last >= first); <---------- Here

        swap(list, pivotIndex, first);

        return first;
    }

如果必须在 index[length/2] 中发生更改，则最终会导致未处理。

输出示例

Output : 0
[16, 40, 44, 60, 82, 5, 72, 63, 67, 89, 80, 31, 23, 44, 81]
[5, 16, 23, 31, 40, 44, 44, 60, 63, 67, 72, 80, 81, 82, 89]

Output : 1
[89, 28, 69, 77, 46, 79, 40, 66, 40, 4, 59, 9, 95, 92, 83]
[4, 9, 28, 40, 40, 46, 59, 66, 69, 77, 79, 83, 89, 92, 95]

Output : 2
[74, 54, 12, 42, 95, 29, 74, 76, 24, 18, 45, 37, 14, 87, 19]
[12, 14, 18, 19, 24, 29, 37, 42, 45, 54, 74, 74, 76, 87, 95]

Output : 3
[1, 48, 26, 42, 25, 79, 32, 31, 82, 65, 71, 43, 15, 26, 49]
[1, 15, 25, 26, 26, 31, 32, 42, 43, 48, 49, 65, 71, 79, 82]

Output : 4
[21, 6, 99, 30, 41, 93, 85, 10, 35, 67, 43, 4, 88, 62, 18]
[4, 6, 10, 18, 21, 30, 35, 41, 43, 62, 67, 85, 88, 93, 99]

Output : 5
[54, 62, 22, 22, 51, 71, 32, 17, 26, 19, 94, 61, 53, 82, 96]
[17, 19, 22, 22, 26, 32, 51, 53, 54, 61, 62, 71, 82, 94, 96]

【讨论】：