門松列 is_kadomatsu

yukicoderでよく使うのでメモ

bool is_kadomatsu(ll a, ll b, ll c){
    if(a==b || b==c || a==c){
        return false;
    }
    ll diff0 = b-a;
    ll diff1 = c-b;
    if(diff0*diff1<0){
        return true;
    }else{
        return false;
    }
}