POJ 2115(扩展欧几里德定理)

题目链接:http://poj.org/problem?id=2115

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<queue>
using namespace std;

const int INF=0x3f3f3f3f;
const int maxn=1010;

typedef __int64 LL;
LL A,B,C,k;

LL Extended_Euclid(LL a,LL b,LL &x,LL &y){
	if(b==0){
		x=1;
		y=0;
		return a;
	}
	LL d=Extended_Euclid(b,a%b,x,y);
	LL tmp=x;
	x=y;
	y=tmp-a/b*y;
	return d;
}

int main(){
#ifndef ONLINE_JUDGE
    freopen("test.in","r",stdin);
    freopen("test.out","w",stdout);
#endif
	while(~scanf("%I64d%I64d%I64d%I64d",&A,&B,&C,&k)){
		if(!A&&!B&&!C&&!k) break;
		LL x,y,n=(LL)1<<k;
		LL d=Extended_Euclid(C,n,x,y);
		if((B-A)%d!=0) puts("FOREVER");
		else{
			x=(x*((B-A)/d))%n;
			x=(x%(n/d)+n/d)%(n/d);
			cout<<x<<endl;
		}
	}
	return 0;
}


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