POJ 2115 - C Looooops（扩展欧几里得）

题目链接：https://blog.csdn.net/shengtao96/article/details/51234355

AC代码：

#include<iostream>
#include<cmath>
# define INF 0x7fffffff
# define ll long long
using namespace std;
ll exgcd(ll a,ll b ,ll &x,ll &y)
{
    if(!b)
    {
        x=1;
        y=0;
        return a;
    }
    ll ans=exgcd(b,a%b,x,y);
    ll t=x;
    x=y;
    y=t-a/b*y;
    return ans;
}
int main()
{
    ll A,B,C,k;
    while(cin>>A>>B>>C>>k&&A+B+C+k)
    {
        ll b=(ll)1<<k;
        ll x,y;
        ll gcd=exgcd(C,b,x,y),cc=B-A;
        if(cc%gcd==0)
        {
            x*=cc/gcd;
            b/=gcd;
            x=(x%b+b)%b;
            cout<<x<<endl;
        }
        else cout<<"FOREVER"<<endl;
    }
    return 0;
}