组合数求法

int quickMul(int x,int k,int mod) {                     //用来求逆元
    if(k == 0) return 1;
    int tmp = quickMul(x,k / 2);
    tmp = (tmp * tmp) % mod;
    if(k & 1) tmp = (tmp * x) % mod;
    return tmp;
}

int C(int n,int k) {
    return frac[n] * quickMul(frac[k],mod - 2) % mod * quickMul(frac[n - k],mod - 2) % mod;
}
    
void solve() {
    frac[0] = 1;
    for(int i = 1;i <= n;i++) frac[i] = frac[i - 1] * i;        //递推求阶乘
}