求第K小的数有一个测试集反复WA，C语言，手敲快排

代码如下

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

#define Cutoff 3

void swap(long long* a, long long* b)
{
    long long temp;
    temp = *a;
    *a = *b;
    *b = temp;
}

void sort(long long arr[], int n)
{
    int i, j;
    for (i = 1;i < n;i++)
    {
        long long temp = arr[i];
        for (j = i;j > 0 && arr[j - 1] > temp;j--)
        {
            arr[j] = arr[j - 1];
        }
        arr[j] = temp;
    }
}

long long Median3(long long a[], int left, int right)
{
    int center = (left + right) / 2;
    if (a[left] > a[center])
        swap(&a[left], &a[center]);
    if (a[left] > a[right])
        swap(&a[left], &a[right]);
    if (a[center] > a[right])
        swap(&a[center], &a[right]);
    swap(&a[center], &a[right - 1]);
    return a[right - 1];
}

long long quicksort1(long long a[], int left, int right,int k)//
{
    int i, j;
    long long pivot;
    if (left + Cutoff <= right)
    {
        pivot = Median3(a, left, right);
        i = left;
        j = right - 1;
        for (;;)
        {
            while (a[++i] < pivot) {}
            while (a[--j] > pivot) {}
            if (i < j)
                swap(&a[i], &a[j]);
            else
                break;
        }
        swap(&a[i], &a[right - 1]);
        if (i == k)
            return a[i];
        if(i<k)
            return quicksort1(a, i + 1, right,k);
        else if(i>k)
            return quicksort1(a, left, i - 1,k);
    }
    else
        sort(a, right - left + 1);
    return a[k];
}

int main()
{
    long long a[5000000] = { 0 };
    int n, k;
    scanf("%d %d", &n, &k);
    for (int i = 0;i < n;i++)
    {
        scanf("%lld", &a[i]);
    }

    printf("%lld", quicksort1(a, 0, n - 1,k));
    return 0;
}

用的三分中值取快排的枢纽元并且递归时筛选k所在位置，不可取时用插入排序排序返回第K小的数字 仅有第五个测试集WA，错误信息如下 Wrong Answer.wrong answer On line 1 column 6, read 0, expected 1.