nefu84 - 五指山

扩展欧几里德算法

列方程dt-ln = y-x

求基础解，在求最小t解

代码如下：

#include <cstdio>
void gcd(long long a, long long b,long long &d,long long &x, long long &y)
{
    if(b==0) {x = 1; y = 0; d = a;}
    else {gcd(b,a%b,d,y,x); y-=(a/b)*x;}
}
int main ()
{
    long long n, d, x, y;
    int cas;
    scanf("%d",&cas);
    while(cas--)
    {
        scanf("%lld%lld%lld%lld",&n,&d,&x,&y);
        long long a = d, b = n, c = y-x, dg;
        gcd(a,b,dg,x,y);
        if(c%dg) printf("Impossible\n");
        else
        {
            long long g = c/dg, k = b/dg;
            x *= g;
            printf("%lld\n",(x%k+k)%k);
        }
    }
    return 0;
}