规则Rules：
填入1~4，使得每行、列、宫内数字都是1~4不重复，并且余下两个空格（不填入任何字符）。外提示是这一方向上的第一个数字。
Write1~4, make each Row/Column/Box contains a group of 1~4, and two blank cells with nothing inside. The outside clue is the first number on this direction.
◆在线做 Do it by CTC
◆在线做 Do it by Penpa
