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 = ;

 typedef long long ll;

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

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

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