POJ 2115 C Looooops

大意不再赘述。

思路：简单的一元线性同余方程，有点KD，移位运算符强制转换行不通，必须用1LL<<K才可以。

#include <iostream>#include <cstdlib>#include <cstdio>#include <cstring>#include <string>#include <cmath>using namespace std;typedef __int64 LL;LL A, B, C;int K;void ex_gcd(LL a, LL b, LL &d, LL &x, LL &y){if(!b){d = a; x = 1; y = 0;}else{ex_gcd(b, a%b, d, y, x);y -= x*(a/b);}}void solve(){LL d, x, y;ex_gcd(C, 1LL<<K, d, x, y);if((B-A) % d)  { printf("FOREVER\n"); return ;}LL b1 = (1LL<<K) / d;    x *= (B-A) / d;LL ans = (x % b1 + b1) % b1;printf("%I64d\n", ans);}int main(){while(scanf("%I64d%I64d%I64d%d", &A, &B, &C, &K) && (A+B+C+K)){solve();}return 0;}