c++ - 如果有重复值，快速排序无限循环

Question

我有一个快速排序程序，在我尝试对具有重复数字的数组进行排序之前效果很好。程序陷入无限循环。我相信这发生在 While(lower < upper)代码块。

void quickSort(int array[], int size){
  if(size < 2) return;

  int pivot, lower, upper, temp;

  //Set the indeces for the first and last elements                                 
  lower = 0;
  upper = size - 1;

  //Select pivot element randomly                                                   
  pivot = array[rand() % (size)];                                                                                                     

  while(lower < upper){
    //Lower must be a number < than pivot and upper a number >= pivot               
    while(array[lower] < pivot){
      lower++;
    }
    while(array[upper] > pivot){
      upper--;
    }

    //Swap upper and lower                                                          
    temp = array[lower];
    array[lower] = array[upper];
    array[upper] = temp;
  }

  //Repeat the past actions on the two partitions of the array recursively          
  quickSort(array, lower);
  quickSort(&array[lower+1], size-lower-1);
}

Answer 1

编辑:添加代码。

来自 Wikipedia ，就地快速排序的伪代码如下:
(不是说要盲目信任)

function quicksort(array)
    if length(array) > 1
        pivot := select any element of array
        left := first index of array
        right := last index of array
        while left ≤ right
            while array[left] < pivot
                left := left + 1
            while array[right] > pivot
                right := right - 1
            if left ≤ right
                swap array[left] with array[right]
                left := left + 1
                right := right - 1
        quicksort(array from first index to right)
        quicksort(array from left to last index)

因此您看到它与您的算法非常相似，只是稍作修改。

while(lower <= upper){

此外，只有在 lower <= upper 时才需要交换然后更新索引。

并且您的代码在递归调用方面有所不同:

quicksort(array from first index to right) {array[0] to array[upper]}
quicksort(array from left to last index) {array[lower] to array[size-1]}

这是因为现在它已经退出 while 循环，upper 小于 lower。

完整的工作代码:

#include <iostream>
#include <cstdlib>
using namespace std;

void quickSort(int array[], int size){
  if(size < 2) return;

  int pivot, lower, upper, temp;

  //Set the indeces for the first and last elements                                 
  lower = 0;
  upper = size - 1;

  //Select pivot element randomly                                                   
  pivot = array[rand() % (size)];                                                                                                     

  while(lower <= upper){
    //Lower must be a number < than pivot and upper a number >= pivot               
    while(array[lower] < pivot){
      lower++;
    }
    while(array[upper] > pivot){
      upper--;
    }

    //Swap upper and lower                                                          
    if ( lower <= upper ) {
        temp = array[lower];
        array[lower] = array[upper];
        array[upper] = temp;
        lower++;
        upper--;
    }
  }

  //Repeat the past actions on the two partitions of the array recursively          
  quickSort(array, upper+1);
  quickSort(&array[lower], size-lower);
}

int main() {
    // your code goes here
    int myArray[] = { 10, 9, 8, 7, 7, 7, 7, 3, 2, 1};
    quickSort( myArray, 10 );
    for ( int i = 0; i < 10; i++ )
        cout << myArray[i] << " ";
    return 0;
}

输出:

1 2 3 7 7 7 7 8 9 10