快速乘

求两个数 ab mod p，但是ab会爆long long的情况
o(1)

long long multi(long long x,long long y,long long mod)
{
long long tmp=(x*y-(long long)((long double)x/mod*y+1.0e-8)*mod);
return tmp<0 ? tmp+mod : tmp;
}

o(logn)

inline ll mult_mod(ll a, ll b, ll m)
{
    ll res = 0;
    while(b){
        if(b&1) res = (res+a)%m;
        a = (a+a)%m;
        b >>= 1;
    }
    return res;
}