快速幂:
ll ksm(ll a, ll b, ll mod) {
ll ans = 1, base = a;
while(b != 0) {
if(b & 1 ) {
ans = (ans * base) % mod;
}
base = (base * base) % mod;
b >>= 1;
}
return ans%mod;
}

大数乘法：
ll muti(ll a, ll b, ll mod) {
ll ans = 0;
while (b != 0) {
if (b & 1) {
ans += a;
ans %= mod;
}
a <<= 1;
a %= mod;
b >>= 1;
}
return ans;
}