POJ-2115 C Looooops 擴充GCD

這題就是一個簡單擴充GCD，方程為 x*C + y * 2^k = B-A.

代碼如下：

#include <cstdlib>#include <cstdio>#include <cstring>#include <iostream>using namespace std;typedef long long int Int64;Int64 A, B, C, K;Int64 _pow(Int64 a, Int64 b){    Int64 ret = 1;    while (b) {        if (b & 1) {            ret *= a;        }        a *= a;        b >>= 1;    }    return ret;}Int64 ext_gcd(Int64 a, Int64 b, Int64 &x, Int64 &y){    int temp, ret;    if (!b) {        x = 1, y = 0;        return a;    }    ret = ext_gcd(b, a % b, x, y);    temp = x, x = y, y = temp - a/b*y;    return ret;}int main(){    Int64 g, x, y;    while (scanf("%lld %lld %lld %lld", &A, &B, &C, &K), A|B|C|K) {        Int64 a = C, b = _pow(2, K);        g = ext_gcd(a, b, x, y);        if ((B-A)%g != 0) {            puts("FOREVER");            continue;        }        x *= ((B-A)/g);        x %= (b / g);        if (x < 0) {            x += b / g;        }        cout << x << endl;    }    return 0;}