CF817B Makes And The Product

思路：

模拟，数学。

实现：

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;

const int MAXN = 100005;

typedef long long ll;

int a[MAXN], last[3], num[3], n;

ll cnm(int n, int m)
{
    ll s = 1;
    int k = 1;
    if(m > n / 2)
        m = n - m;
    for(int i = n - m + 1; i <= n; i++)
    {
        s *= (ll)i;
        while(k <= m && s % k == 0)
        {
            s /= (ll)k;
            k++;
        }
    }
    return s;
}

int main()
{
        scanf("%d", &n);
        for (int i = 0; i < n; i++)
                scanf("%d", &a[i]);
        sort(a, a + n);
        int cnt = 0;
        last[0] = 0;
        int i = 1;
        while (i < n)
        {
                if (a[i] != a[last[cnt]])
                {
                        num[cnt] = i - last[cnt];
                        last[++cnt] = i;
                }
                if (cnt == 2) break;
                i++;
        }
        while (a[i] == a[last[cnt]] && i < n) i++;
                num[cnt] = i - last[cnt];
        if (num[0] >= 3) cout << cnm(num[0], 3) << endl;
        else if (num[0] == 2) cout << num[1] << endl;
        else if (cnt == 1 || num[1] >= 2) cout << cnm(num[1], 2) << endl;
        else cout << num[2] << endl;
        return 0;
}