How to solve BUG+1 in Sudoku
BUG+1 is a board where every empty cell has two candidates except one with three, and that one cell must take the digit that would otherwise appear three times.
How it works
If every empty cell had exactly two candidates, each digit would have exactly two places in every row, column and box. Then the puzzle could be finished two different ways, so a puzzle with one solution never gets there. This is called a bivalue universal grave, or BUG.
When one cell has three candidates and every other has two, that cell has to break the pattern. Look at its three digits: one of them appears three times in its row, three times in its column and three times in its box. That digit is the answer.
How to spot it
It only turns up late in a puzzle, when almost every note has been narrowed to two. Count the cells that are not down to two; if there is exactly one, with three notes, check whether one of its digits appears three times in each of its row, column and box.
A worked example
This is a step from a real puzzle, a moment where the next move needs BUG+1.
Struck-through digits are the ones that can be removed.
Every empty cell has two candidates except R9C6 (1/2/6). If it were also down to two, each digit would appear exactly twice in every row, column and box, and the puzzle could be completed in two different ways. A puzzle has only one solution, so R9C6 must be the digit that appears three times in its row, column and box: 2.
Where it fits in Rocksled Sudoku
Puzzles that need BUG+1 are never rated below Expert. Each step of this kind adds 76 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.