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.