组合数专题

const int N = 1e5 + 10;
const int MOD = 1e9 + 7;
int f[N], finv[N], inv[N];

void init(void) {
    inv[1] = 1;
    for (int i=2; i<N; ++i) {
        inv[i] = (MOD - MOD / i) * 1ll * inv[MOD%i] % MOD;
    }
    f[0] = finv[0] = 1;
    for (int i=1; i<N; ++i) {
        f[i] = f[i-1] * 1ll * i % MOD;
        finv[i] = finv[i-1] * 1ll * inv[i] % MOD;
    }
}

int comb(int n, int k)  {       //C (n, k) % MOD
    if (k < 0 || k > n) return 0;
    return f[n] * 1ll * finv[n-k] % MOD * finv[k] % MOD;
}

const int N = 2000 + 10;
const int INF = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
int comb[N][N];

void init(void) {
    for (int i=0; i<N; ++i) {
        comb[i][i] = comb[i][0] = 1;
        for (int j=1; j<i; ++j) {
            comb[i][j] = comb[i-1][j] + comb[i-1][j-1];
            if (comb[i][j] >= MOD)  {
                comb[i][j] -= MOD;
            }
        }
    }
    //printf ("%d\n", comb[6][3]);
}