几种模幂算法

#include <iostream.h>
void main()
{
	int x,r,p;
	int a,b,c,l;
	cout<<"x=";
	cin>>x;
	cout<<"r=";
	cin>>r;
	cout<<"p=";
	cin>>p;
	a=x;
	b=r;
	c=1;
	while  (b!=0)
	{
		l=b%2;
		if(l==0)
		{
	       b=b/2;a=a*a%p;
		}
		else 
		{	
			b=b-1;c=(a*c)%p;
        } 
		
	}
	cout<<"c="<<c<<endl;
}