Miller_Rabin算法（随机算法，判断一个数是否是素数）

const int S = 20;//随机算法判定次数，S越大，判错概率越小
LL pow_mod(LL a, LL b, LL mod) { // a^b%mod
    LL ans = 1;
    a = a % mod;
    while(b) {
        if(b & 1) {
            ans = (ans * a) % mod;
        }
        a = ( a * a ) % mod;
        b >>= 1;
    }
    return ans;
}
bool check(LL a, LL n, LL x, LL t) {
    LL ret = pow_mod(a, x, n);
    LL last = ret;
    for(int i = 1; i <= t; i++) {
        ret = (ret * ret) % n;
        if(ret == 1 && last != 1 && last != n - 1) return true;
        last = ret;
    }
    if(ret != 1) return true;
    else return false;
}
bool Miller_Rabin(long long n) {
    if(n < 2)return false;
    if(n == 2) return true;
    if( (n & 1) == 0) return false;
    LL x = n - 1;
    LL t = 0;
    while( (x & 1) == 0 ) {
        x >>= 1;
        t++;
    }
    for(int i = 0; i < S; i++) {
        LL a = rand() % (n - 1) + 1;
        if(check(a, n, x, t))
            return false;
    }
    return true;
}