快速幂 - OI Wiki (oi-wiki.org)

#include<bits/stdc++.h>
using namespace std;
  
int main()
{
    unsigned long long a,b,mode ;
    cin>>a>>b>>mode;
    int sum=1;
    a =a%mode;
    while(b>0)
	{
        if (b%2==1)
		{
			sum =(sum*a)%mode;
		}
        b /= 2;
        a = (a * a) % mode;

   }
cout<<sum;
}