Codeforces Round #309 (Div. 1), problem: (A) Kyoya and Colored Balls

#include <cstdio>
#include <cstdlib>
#include <algorithm>

using namespace std;

typedef long long int int64;

int64 mut[1000001];;

const int64 P = 1000000007;

int64 npow(int64 a, int n){
    if(!n) return 1;
    int64 g = npow(a, n>>1);
    g = (g*g)%P;
    if(n&1) return (g*a)%P;
    return g;
}

int64 rev(int64 n){ return npow(n, P-2);}

int64 C(int64 a, int64 b){
    int64 ret = mut[a];
    ret *= rev(mut[b]); ret %= P;
    ret *= rev(mut[a-b]); ret %= P;
    return ret;
}

int main(){
    mut[0] = 1;
    for(int lx = 1;lx <= 1000000;lx++)
        mut[lx] = (mut[lx-1]*lx)%P;

    int n; scanf("%d", &n);
    int64 ans = 1; int sum = 0;
    for(int lx = 0;lx < n;lx++){
        int k; scanf("%d", &k);
        ans *= C(sum+k-1, sum);
        ans %= P;
        sum += k;
    }
    printf("%d\n", (int)ans);
    return 0;
}