EXgcd

long long exgcd(long long a,long long b,long long&x,long long&y)
{
  if(!b)
  {
    x=1,y=0;
    return a;
  }
  long long gcd=exgcd(b,a%b,y,x);
  y-=(a/b)*x;
  return gcd;
}

我tm再忘记就是sb