拓展歐幾里得算法

青蛙的約會

ll extend(ll a, ll b,ll &x,ll &y)
{
    ll r,t;
    if(!b){
        x = 1; y = 0; return a;
    }
    r = extend(b,a % b,x,y);
    t = x;
    x = y;
    y = t - a / b * y;
    return r;
}