Two rooks attack each other if they share the same row or column, and there are no other rooks between them.

对于像我这种对国际象棋不够熟悉的蒟蒻，只看中文翻译，很容易以为只要在同行或同列就算互相攻击了，再一看$k \leq \frac {n * (n-1)}2$，错误题意就显得更加正确了。。。