NewCoder 排队

排队
在这里插入图片描述
数学

#pragma GCC optimize(2)
#include 
 
#define endl "\n"
#define x first
#define y second
#define pb(a) push_back(a);
#define mst(a, x) memset(a, x, sizeof a)
#define all(a) a.begin(), a.end()
#define complete_unique(a) a.erase(unique(a.begin(), a.end()), a.end())
 
using namespace std;
 
typedef long long LL;
typedef unsigned long long ULL;
typedef pair<int, int> PII;
 
const int INF = 0x3f3f3f3f, MOD = 1e9 + 7;

const int N = 1e5 + 10;

int cnt[N], n;

LL fact[N] = {0}, IV2 = (MOD + 1) / 2;

LL cn2(LL x)
{
    return x * (x - 1) % MOD * IV2 % MOD;
}

void solve() {
    int a;
    cin >> n;
    fact[1] = 1;
    for(int i = 2; i < N; i ++) fact[i] = fact[i - 1] * i % MOD;
    for(int i = 1; i <= n; i ++)
    {
        cin >> a;
        cnt[a] ++;
    }
    LL res = cn2(n);
    for(int i = 1; i <= (int)1e5; i ++)
    {
        res = ((res - cn2(cnt[i]) % MOD) + MOD) % MOD;
    }
    cout << fact[n] * res % MOD *IV2 % MOD << endl;
}
 
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int T = 1;
  // cin >> T;
  while (T--) solve();
 
  return 0;
}
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