POJ 2115 C Looooops 模线性方程（扩展欧几里得）

#include<iostream>
#include<cmath>
using namespace std;

__int64 Egcd ( __int64 a, __int64 b, __int64 &x, __int64 &y )
{
__int64 tmp, ret;
if ( b == 0 )
{
x = 1, b = 0;
return a;
}
ret = Egcd ( b, a%b, x, y );
tmp = x, x = y, y = tmp - a / b * y;
return ret;
}

int main()
{
__int64 A, B, C, K, x, y;
while ( 1 )
{
scanf("%I64d%I64d%I64d%I64d",&A,&B,&C,&K);
if ( A+B+C+K == 0 ) break;
__int64 M = (__int64)pow(2.0,K+0.0);
__int64 a = C, b = M, c = B - A;
__int64 d = Egcd ( a, b, x, y );
if ( c % d != 0 )
{
printf("FOREVER\n");
continue;
}

x = (c*x/d)%(b/d);
if ( x < 0 )
x += b/d;
printf("%I64d\n",x);
}
return 0;
}