【模板】快速乘 （解决两个数相乘爆long long)

原文章：https://blog.youkuaiyun.com/anxdada/article/details/76855632 

 

long级:

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;
}

 O(1)级:

inline ll mult_mod(ll a, ll b, ll m)
{
    ll c = a*b-(ll)((long double)a*b/m+0.5)*m;
    return c<0 ? c+m : c;  //就是算的a*b%m;
}