POJ 3641 pseudoprime Numbers (fast Power + prime)

Title Link: http://poj.org/problem?id=3641

Test instructions: Given a,p, if p is a prime number and AP = A (mod p) that is, a P-modulo remainder is a, output "yes", otherwise output "no".

Solving: fast Power + judging primes.

Code:

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long LL;
BOOL IsPrime (int n)
{
	bool flag=true;
	for (int i=2;i*i<n;i++)
	{
		if (n%i==0)
		{
			flag=false;
			break;
		}
	}
	return flag;
}
ll POW (ll x,ll n,ll MoD)
{
	ll res=1;
	while (n>0)
	{
		if (n&1) res=res*x%mod;
		X=x*x%mod;
		n>>=1;
	}
	return res;
}
int main ()
{
	//freopen ("In.txt", "R", stdin);
	Freopen ("OUT.txt", "w", stdout);
	int p,a;
	while (scanf ("%d%d", &p,&a))
	{
		if (p==0&&a==0) break;
		if (POW (a,p,p) ==a&&!isprime (p))
		{
			printf ("yes\n");
		}
		else 
			printf ("no\n");
	}
	return 0;
}