sudokUI › Sudoku techniques › Skyscraper
Skyscraper in sudoku
Turbot Fish shape · 4-candidate X-Chain · two Sashimi X-Wings · how it fits
Two rows (or columns) each hold a conjugate pair for digit X; one end of each shares a column (or row), and the other ends do not see each other.
Why it works
The two ends that share a column (or row) cannot both be X, so at least one of the other two ends must be X. X is eliminated from every cell outside the pattern that sees both of those ends.
How to spot it
Pick a digit, find two rows where it has exactly two places, and check whether two of those places sit in the same column.
Worked example
Skyscraper on 1: r2c3 and r2c7 are the only places for 1 in row 2, as are r9c3 and r9c9 in row 9, and r2c3 sees r9c3. If r2c7 is not 1, r2c3 is, so r9c3 is not, so r9c9 is: one of r2c7 and r9c9 is 1, and 1 is removed from every cell that sees both. So 1 is removed from r1c9, r8c7.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Skyscraper and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs Skyscraper, the puzzle's difficulty rating grows by 130. Its class is Hard. It is needed in 1 in 11 puzzles that sudokUI generates. How the rating works.