选择出数列中前k个最大的数。

这里因为数据特殊，所以可以使用hash表的方法：

#include <cstdio>
#include <algorithm>
#include <stdlib.h>
#include <limits>
using namespace std;

const int SIZE = 1000005;
const int SMALL = -500000;
bool arr[SIZE];

int main()
{
	int n, m, a, maxN = INT_MIN;
	while (~scanf("%d %d", &n, &m))
	{
		for (int i = 0; i < n; i++)
		{
			scanf("%d", &a);
			arr[a - SMALL] = true;
			if (a - SMALL > maxN) maxN = a - SMALL;
		}
		for (int i = maxN; i >= 0 && m; i--)
		{
			if (arr[i])
			{
				if (m > 1) printf("%d ", i + SMALL);
				else printf("%d\n", i + SMALL);
				m--;
			}
		}
	}
	return 0;
}

#include <cstdio>
#include <algorithm>
#include <stdlib.h>
#include <time.h>
using namespace std;

int partition(int *arr, int low, int up)
{
	for (int i = low; i < up; i++)
		if (arr[i] >= arr[up]) swap(arr[low++], arr[i]);

	swap(arr[low], arr[up]);
	return low;
}

//K作为绝对下标位置处理
void randSelectK(int *arr, int low, int up, int K)
{
	int r = low + rand() % (up - low + 1);
	swap(arr[r], arr[up]);

	int idx = partition(arr, low, up);
	if (idx > K) randSelectK(arr, low, idx-1, K);
	else if (idx < K) randSelectK(arr, idx+1, up, K);
}

void randQuickSort(int *arr, int low, int up)
{
	if (low >= up) return ;//不能是low == up

	int r = low + rand() % (up - low + 1);
	swap(arr[r], arr[up]);

	int mid = partition(arr, low, up);
	randQuickSort(arr, low, mid-1);
	randQuickSort(arr, mid+1, up);
}

const int SIZE = 1000000;
int arr[SIZE];	
int main()
{
	int n, m;
	srand(unsigned(time(NULL)));
	while(~scanf("%d %d", &n, &m))
	{
		for (int i = 0; i < n; i++)
			scanf("%d", &arr[i]);

		randSelectK(arr, 0, n-1, m-1);
		randQuickSort(arr, 0, m-1);
		for (int i = 0; i < m-1; i++)
		{
			printf("%d ", arr[i]);
		}
		printf("%d\n", arr[m-1]);
	}
	return 0;
}