pku1061 青蛙的约会

http://poj.org/problem?id=1061

数论，单变元模线性方程

#include <stdio.h>

long long extend_gcd(long long a, long long b, long long &x, long long &y)
{
    if(b == 0)
    {
        x = 1;
        y = 0;
        return a;
    }
    else
    {
        long long r = extend_gcd(b, a%b, y, x);
        y -= x*(a/b);
        return r;
    }
}

long long line_mod_equation(long long a, long long b, long long n)
{
    long long x, y;
    long long d = extend_gcd(a, n, x, y);
    if(b % d == 0)
    {
        x %= n;
        x += n;
        x %= n;
        x = (x*(b/d) % (n/d));
        x = (x%(n/d) + (n/d)) % (n/d);
        return x;
    }
    else
    {
        return -1;
    }
}

int main()
{
    long long a, b, n, temp;
    long long X, Y, M, N, L;
    scanf("%lld%lld%lld%lld%lld", &X, &Y, &M, &N, &L);
    a = N - M;
    b = X - Y;
    n = L;
    temp = line_mod_equation(a, b, n);
    if(temp == -1)
    {
        printf("Impossible\n");
    }
    else
    {
        printf("%lld\n", temp);
    }
    return 0;
}