poj 1061 青蛙的约会

题目：http://poj.org/problem?id=1061

思路：扩展欧几里得

#include <iostream>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <cstdio>
using namespace std;
long long gcd(long long a,long long b)
{
    if(b==0)
        return a;
    return gcd(b,a%b);
}
long long exgcd(long long a,long long b,long long &x,long long &y)
{
    if(b==0)
    {
        x=1;
        y=0;
        return a;
    }
    else
    {
        long long ans=exgcd(b,a%b,x,y);
        long long t=x;
        x=y;
        y=t-a/b*y;
        return ans;
    }
}
int main()
{
    long long x,y,m,n,l;
    ///  (x+mt) - (y+nt) = kl
    // (x-y) + (m-n)t = kl
    //  (m-n)t - kl = y-x
    cin>>x>>y>>m>>n>>l;
    long long xx,yy;
    long long A=-(n-m),B=-l,C=-x+y;
    long long g=gcd(A,B);
    if(C%g!=0)
        cout<<"Impossible
";
    else
    {
        A/=g;
        B/=g;
        C/=g;
        exgcd(A,B,xx,yy);
        xx*=C;
        xx=(xx%B+B)%B;
        cout<<xx<<endl;
    }
    return 0;
}