using namespace std;
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<ctime>
#include<map>
#include<set>
#include<vector>
#include<stack>
#include<queue>
#include<numeric>
#include<algorithm>
 
#define FOR(i, a, b) for(int i = (a), _b = (b); i < b; ++i)
#define FORE(i, a, b) for(int i = (a), _b = (b); i <= b; ++i)
#define FORD(i, a, b) for(int i = (a), _b = (b); i >= b; --i)
 
#define RI ({int n; scanf(" %d ", &n); n;})
#define BG begin()
#define ED end()
#define ST first
#define ND second
#define MP make_pair
#define PB push_back
 
#define ITR(c) typeof(c.BG)
#define TR(c, it) for(ITR(c) it = c.BG; it != c.ED; ++it)
#define SZ size()
#define EXIST(c, x) ( c.find(x) != c.ED )
 
typedef long long LL;
typedef pair<int, int> II;
typedef pair<int, II> III;
typedef vector<int> VI;
typedef vector<II> VII;
typedef vector<VI> VVI;
typedef vector<VII> VVII;
 
const int MAXN = 2+10000;
const int K = 1000000007;
const int INF = int(1e9);
int n;
int a[MAXN];
int main(){
    scanf("%d",&n);
    FOR(i,0,n) scanf("%d",&a[i]);
    sort(a,a + n);
    int kq = 1;
    FOR(i,0,n){
        kq *= (a[i] - i);
        if (kq >= K) kq = kq % K;
        }
    printf("%d",kq);
    return 0;
}

- Đầu tiên sắp xếp lại dãy số (dùng Quick Sort).
- res := 1;
  for i := 1 to n do
    res := res * (a[i] - i + 1) mod k;
(Chú ý biến khai báo biến res là Int64).