POJ-2115 C Looooops (模线性方程)

 1 #include <bits/stdc++.h>
 2 
 3 using namespace std;
 4 typedef long long LL;
 5 LL A,B,C,k;
 6 LL a,b,c,d,e,f,x,y;
 7 LL exgcd(LL a,LL b,LL &x,LL &y){
 8     if (b==0)
 9     {x=1;
10      y=0;
11      return a;
12     }
13     LL d=exgcd(b,a%b,x,y);
14     LL t=x;
15     x=y;
16     y=t-(a/b)*y;
17     return d;
18 }
19 int main(){
20     freopen ("c.in","r",stdin);
21     freopen ("c.out","w",stdout);
22     int i,j;
23     while (1)
24     {scanf("%lld%lld%lld%lld",&A,&B,&C,&k);
25      if (A==0 && B==0 && C==0 && k==0)
26       break;
27      a=C,b=1ll<<k,c=B-A;
28      d=exgcd(a,b,x,y);
29      if (c==0)
30      {puts("0");
31       continue;
32      }
33      if (c%d!=0)
34      {puts("FOREVER");
35       continue;
36      }
37      e=x*(c/d);
38      f=(e%(b/d)+b/d)%(b/d);
39      printf("%lld\n",f);
40     }
41     return 0;
42 }