快速乘/龟速乘模板

用于防止大数乘法溢出

//龟速乘O(logn)
int gsc(int a,int b,int p){
    int res=0;
    while(b){
        if(b&1)res=(res%p+a%p)%p;
        a=(a<<1)%p;
        b>>=1;
    }
    return res;
}
//快速乘O(1)
ll ksc(int a,int b,int p){
    ll z = (long double)a/p*b;
    ll res =(ull)a*b -(ull)(z*p);
    return (res+p)%p;
}