c - 为什么我的快速排序算法不适用于重复元素？

Question

我用 C 实现了快速排序算法来对数组的元素进行排序。它适用于所有情况，除非数组具有两个或多个相等元素。我一直在尝试修复它并一直在调试它，但当存在重复元素时，我似乎无法让它工作。

如果您能帮助我更改代码以使其也适用于重复元素，我将不胜感激。

#include <stdio.h>
#include <stdlib.h>

//Random Array Length
#define L 10
#define MAX 100

void smarter_sort(int[],int,int);
void swap(int[],int,int);
int choose_piv(int[],int,int);

int main(){

    int i, a[L];

    //Generate an array of random numbers
    for(i=0; i<L; i++)
        a[i]= rand() % (MAX+1);

    //Unsorted Array
    printf("\nUnsorted array: ");
    for(i=0; i<L; i++)
            printf("%d    ", a[i]);

    //Sorted Array
    smarter_sort(a,0,L-1);
    printf("\nSorted array:   ");
            for(i=0; i<L; i++)
                printf("%d    ", a[i]);
    return 0;
}

//Recursively defined quicksort (Pseudo-code listing 1.9)
void smarter_sort(int a[], int l, int r){
    if(r > l){
        int piv = choose_piv(a, l, r);
        smarter_sort(a, l, piv-1);
        smarter_sort(a, piv+1, r);
    }
}

//Swap Elements
void swap(int a[], int i, int j){
    int t=a[i];
    a[i]=a[j];
    a[j]=t;
}

//Choosing the pivot (pseudo-code listing 1.10)
int choose_piv(int a[], int l, int r){
    //defining pointers and pivot
    int pL = l, pR = r;
    int piv = l;

    while (pL < pR){
        //finding the first left element greater than piv
        while(a[pL] < a[piv])
            pL++;

        //finding the first right element greater than piv
        while(a[pR] > a[piv])
            pR--;

        //swapping if the pointers do not overlap
        if(pL < pR)
            swap(a, pL, pR);

        if(a[pL]==a[piv]||a[pR]==a[piv]){
            pL++;
            pR--;
        }
    }
    //swapping and returning the rightmost pointer as the pivot
    swap(a, piv, pR);
    return pR;
}

Answer 1

这是您修改后的代码，以便即使数组包含相同元素也能正常工作:

 #include <stdio.h>
 #include <stdlib.h>

 //Random Array Length
 #define L 10
 #define MAX 100

 void smarter_sort(int[],int,int);
 void swap(int[],int,int);


 int main(){

    int i, a[L];

    //Generate an array of random numbers
    for(i=0; i<L; i++)
       a[i]= rand() % (MAX+1);

    //Unsorted Array
    printf("\nUnsorted array: ");
    for(i=0; i<L; i++)
        printf("%d    ", a[i]);

    //Sorted Array
    smarter_sort(a,0,L-1);
    printf("\nSorted array:   ");
        for(i=0; i<L; i++)
            printf("%d    ", a[i]);
    return 0;
 }

 //Recursively defined quicksort (Pseudo-code listing 1.9)
 void smarter_sort(int arr[], int left, int right) {
      int i = left, j = right;
      int pivot = arr[(left + right) / 2];


      while (i <= j) {
           while (arr[i] <pivot)
                i++;
           while (arr[j]>pivot)
                j--;
           if (i <= j) {
                swap(arr,i,j);
                i++;
                j--;
           }
     };


    if (left < j)
          smarter_sort(arr, left, j);
    if (i < right)
          smarter_sort(arr, i, right);
 }


//Swap Elements
void swap(int a[], int i, int j){
    int t=a[i];
    a[i]=a[j];
    a[j]=t;
}