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Medium technique

Last Remaining Cell: Only One Spot Left

Also called a hidden single, or the last remaining cell. In Bubble Sudoku it is a Hidden Single: Last Remaining.

Follow one color across the board. That color cannot repeat in any row or column it is already in. Often that leaves just one cell in a block where it can go.
1

Track one color

BEFORE

Pick a color and follow it. One Red sits just above this block, in the column that runs down its left side. Another Red sits just to its right, in its top row. The block has no Red yet.

2

See what the Reds block

Red cannot repeat in a row, so no cell in the top row of this block can take Red. Red cannot repeat in a column either, so the left column is blocked too. That rules out five empty cells. Three other cells already hold bubbles.

3

Place Red in the last cell

AFTER

Only one cell in the block is still open, so Red must go there. Place it, and the row, column, and block light up in Red, just like in a real game. You did not need a single note. The two Reds already on the board showed you the answer.

Cells being readColor tints match the color being followed

Key Insight

You will use this technique more than any other on Easy and Medium boards. Pick a color that appears many times and check one block at a time. Rule out every cell that shares a row or column with that color. Often only one cell is left. Remember that columns block a color just as well as rows do.

More medium techniques

If you’re stuck on a puzzle, see what to try next, in order.

Bubble Sudoku’s hints walk you through Hidden Single: Last Remaining on a real board, one step at a time.

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