规则Rules：
标准数独基础上，箭头格内数字是这一方向上的摩天楼数。
摩天楼数：从这一方向上看过去，当大数字遮住小数字时，可以看见的数字的个数。
Classic rules applied. Additionally, the arrow cell is the skyscraper number in this direciton.
The skyscraper number means how many numbers can be seen, when you look from this direction, with large numbers hiding the lower ones.
