THE SHORT VERSION
Sudoku: a 9×9 grid, mostly empty. Place digits 1–9 so every row, column, and 3×3 box contains each digit once. You construct a valid arrangement from scratch.
Hitori: an N×N grid, fully pre-filled with digits. Shade some cells so that no digit appears more than once in any row or column, shaded cells never touch each other orthogonally, and all unshaded cells form one connected region. You correct an invalid arrangement by removal.
Sudoku is additive — you build. Hitori is subtractive — you prune.
SIDE BY SIDE
Starting state
Sudoku: mostly empty — you fill cells
Hitori: fully filled — you shade cells out
Constraint structure
Sudoku: one rule applied three ways (row / column / box)
Hitori: three separate rules (no-repeat / no-touch / connectivity)
Box constraint
Sudoku: yes — nine 3×3 boxes, each digit once
Hitori: no — only rows and columns
Arithmetic required
Sudoku: none
Hitori: none
Grid size
Sudoku: always 9×9
Hitori: 5×5 through 8×8 (larger grids available)
End state
Sudoku: every cell filled with a digit
Hitori: some cells shaded, survivors form one connected region
HOW HITORI'S THREE RULES INTERACT
Sudoku has one rule applied uniformly: no digit repeats in any row, column, or box. The simplicity is deceptive — three overlapping units create the complexity. But the constraint type is the same everywhere.
Hitori's three rules create a cascade. Shading one cell to fix a no-repeat violation (rule 1) immediately forces the four neighbours of that cell to be unshaded (rule 2 — shaded cells don't touch). Each newly confirmed-unshaded cell might expose a duplicate elsewhere, forcing another shade. Throughout, rule 3 (connectivity) acts as a global check: any shade that would cut the unshaded region in two is invalid.
This cascade is Hitori's signature. A single correct shade can unlock five or six forced moves in sequence — something Sudoku only achieves with advanced multi-step techniques at high difficulty.
WHAT TRANSFERS FROM SUDOKU
The row/column uniqueness instinct transfers directly. Sudoku trains you to spot digits that appear more than once in a unit — in Hitori, that's exactly the same first scan. Look for any digit that appears twice in a row or column; one of those cells must be shaded.
The no-guessing discipline transfers too. Both puzzles are solvable by pure deduction. If you find yourself guessing in Hitori, a forced move exists — usually a connectivity check that rules out one of the two duplicate candidates.
Sudoku players who struggle with Hitori usually underuse the connectivity rule. It feels like a late-game confirmation check, but in practice it fires early and often — especially on edge cells with limited neighbours.
WHAT DOESN'T TRANSFER
The 3×3 box constraint doesn't exist in Hitori. Pointing pairs, box-line reduction, X-wings — all the techniques built on Sudoku's box structure — have no equivalent here.
The candidate-management approach changes fundamentally. In Sudoku you pencil candidates into cells and eliminate one by one. In Hitori you reason about which cells to shade, not which values to place — the cell already has a value. You're deciding whether to keep or remove, not what to put there.
The spatial/connectivity thinking Hitori requires is genuinely new for most Sudoku solvers. Checking whether a shade would disconnect the unshaded region requires visualising paths across the grid — a skill Sudoku never develops.
WHICH IS HARDER?
At small sizes, Hitori is easier to start. On a 5×5 grid, visible duplicates are immediately obvious and the cascade resolves quickly. There's no 9×9 minimum that makes even easy Sudoku require significant scanning.
At hard difficulty, Hitori is more demanding for most Sudoku players because of the connectivity rule. Checking global path connectivity is harder than checking local row/column/box uniqueness. Expert Sudoku solvers plateau on hard Sudoku faster than hard Hitori, because Hitori's three-rule interaction keeps producing novel forcing patterns that don't appear in Sudoku at all.
WHEN TO MAKE THE SWITCH
If you play Sudoku: try Hitori when you want to use your duplicate-detection instinct in a completely different frame. The opening scan is familiar; the shading cascade and connectivity check are genuinely new skills that will feel different within the first puzzle.
If you play Hitori: try Sudoku when you want a placement puzzle with a third constraint layer (the 3×3 box) that creates long elimination chains. Sudoku's triple-constraint structure fires differently from Hitori's three-rule cascade.