Mini Sudoku · 4 min
Mini Sudoku: Check Three Constraints
Use row, column, and two-by-two box constraints before placing each number.
Last updated: 2026-07-22 · Written and tested by Starbox EditorialList what a unit is missing
Each row, column, and two-by-two box must contain 1, 2, 3, and 4 once. For a row showing 1, blank, 3, blank, the missing set is {2,4}. Do not guess between them yet; carry that small candidate set to the column check.
Intersect all three constraints
A cell is valid only if its number is absent from the row, column, and box. If the row allows {2,4}, the column already contains 4, and the box allows both, the cell must be 2. This intersection is the core operation of mini Sudoku.
Place forced values first
Scan for a unit with only one missing number, then for a cell whose candidate set has one value. Every forced placement updates three units and may create another forced move. Filling the easiest certainty first is faster and safer than working left to right.
Recover from a contradiction
If a later cell has no possible value, an earlier entry was wrong. Review user-entered numbers in reverse order and test each against its row, column, and box. Avoid replacing multiple values at once; one controlled correction preserves the deductions that remain valid.
Return to the game with a clear method and compare like-for-like attempts.