什么是45法则，它如何在这里应用？
最后更新
45规则在杀手数独中最著名，但它在三明治数独中有一个简洁且未被充分利用的应用：它将每个外部线索转化为关于三明治未覆盖单元格的第二个补充线索。
为什么每条线之和为45
每行和每列恰好包含一次数字1到9，且1+2+3+4+5+6+7+8+9 = 45。这是每个有效数独行的固定算术事实。将任何行分成三部分——1和9（和为10）、夹心单元格（和为线索）以及夹心之外的所有东西——这三部分必须构成完整的45。
外部和技巧
Rearranging gives: outside digits = 45 − 10 − clue = 35 − clue. A row with clue 12 has outside digits summing to 23; a row with clue 30 has outside digits summing to just 5. The outside region is a 'shadow sandwich' - A set of distinct digits from 2-8 with a known total - And it obeys exactly the same combination logic as the sandwich itself. When the clue is large, the outside sum is small and often more constraining than the inside: an outside total of 5 across two cells must be {2,3}, immediately fixing two cells at the edges of the line.
同时使用两侧
Work whichever side of the line is tighter. With clue 30, either the sandwich is five cells of {4,5,6,7,8} with {2,3} outside, or six cells with the single digit 5 outside - Two crisp scenarios you can test against the boxes. The 45 rule also provides a permanent error check: at any point, placed digits inside the sandwich may never exceed the clue, and placed digits outside may never exceed 35 − clue. If either total is breached, a placement upstream is wrong.