快速幂

快速幂取模就是在O(logn)内求出a^n mod b的值。

原理：a^b mod n=(a mod n)(b mod n)mod n

 

 

// 快速幂 (a^b)%n 
ll mod_mul(ll a, ll b, ll n){
    ll res = 0;
    while (b){
        if(b & 1) res = (res + a) % n;
        
        a = (a + a) % n;
        b >>= 1;
    }
    return res;
}

ll mod_exp(ll a, ll b, ll n){
    ll res = 1;
    while(b){
        if(b & 1) res = mod_mul(res, a, n);
        
        a = mod_mul(a, a, n);
        b >>= 1;
    }
    return res;
}