扩展欧几里得模板

long long x,y,gcd;
void extend_gcd(long long  a, long long b)
{
    if(b == 0)
    {
        x = 1;
        y = 0;
        gcd = a;
    }
    else {
        extend_gcd(b, a%b);
        long long temp = x;
        x = y;
        y = temp - a/b*y;
    }
}

为了保证求出的x是正整数，通常需要加上：x=(x%mod+mod)%mod