BZOJ 1477 青蛙的约会

exgcd。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
long long p,q,m,n,l,x,y,a,b,c;
long long gcd(long long a,long long b)
{
    if (b==0) return a;
    return gcd(b,a%b);
}
void exgcd(long long a,long long b,long long &x,long long &y)
{
    if (b==0) {x=1;y=0;return;}
    exgcd(b,a%b,x,y);
    long long t=x;x=y;y=t-a/b*y;
}
int main()
{
    scanf("%lld%lld%lld%lld%lld",&p,&q,&m,&n,&l);
    a=m-n;b=l;c=q-p;
    long long d=gcd(a,b);
    if (c%d) printf("Impossible\n");
    else
    {
        a/=d;b/=d;c/=d;
        exgcd(a,b,x,y);
        b=abs(b);x=((c*x)%b+b)%b;if (!x) x+=b;
        printf("%lld\n",x);
    }
    return 0;
}