(动态规划 + 双指针)

C++ 代码

#include <iostream>
#include <algorithm>

using namespace std;

const int N = 5010;

int n, m;
int w[N];
int f[N][N];

int main()
{
    cin >> n >> m;
    for (int i = 1; i <= n; i ++ ) scanf("%d", &w[i]);
    sort(w + 1, w + n + 1);

    for (int i = 1, k = 1; i <= n; i ++ )
    {
        while(w[i] - w[k] > 5) k ++;
        for (int j = 1; j <= m; j ++ )
            f[i][j] = max(f[i - 1][j], f[k - 1][j - 1] + (i - k + 1));
    }

    cout << f[n][m] << endl;
    return 0;
}