Sukaku Sudoku: Rules and Solving Strategy

Sukaku Sudoku starts with candidates, not givens

Sukaku Sudoku follows the same final rules as classic Sudoku: every row, every column, and every 3x3 box must contain the digits 1 through 9 exactly once. The difference is the starting information. Instead of seeing a handful of solved digits, you see one or more allowed candidates printed in every cell. A missing digit in a cell is already forbidden there, even if ordinary row, column, and box logic would otherwise allow it. That makes the printed candidate sets part of the puzzle itself. Do not add a candidate simply because it looks legal under standard Sudoku. Your task is to reduce the supplied lists until every cell has one value. Begin with the smallest lists. A one-candidate cell is solved immediately, while a two-candidate cell is a useful anchor for pairs, wings, and chains. After each placement, remove that digit from all peers just as you would in a normal candidate grid.

Use set logic to solve Sukaku Sudoku cleanly

The strongest early progress in Sukaku Sudoku often comes from naked and hidden subsets because the puzzle has already done the first round of candidate generation for you. If two cells in one row contain only 2 and 7, those digits are reserved for the pair and can be removed from other cells in that row. Hidden pairs work in the opposite direction: if 3 and 8 appear nowhere else in a box, keep only 3 and 8 in the two cells that can take them. Locked candidates are equally valuable. When every remaining 5 in a box lies in one row, 5 can be removed from the rest of that row outside the box. The key habit is to trust the candidate state but keep it current. A stale candidate can hide a single; an invented candidate can destroy the intended logic. When the grid becomes dense, scan one digit at a time so you can compare all of its remaining positions without being distracted by the other marks.

Treat Sukaku Sudoku as candidate training for harder puzzles

Sukaku Sudoku is especially useful if you want to become more comfortable with advanced candidate reasoning. Because there may be few or no traditional givens, you cannot rely on the visual rhythm of solved numbers to guide every move. You learn to notice strong links, bivalue cells, fish patterns, and chains directly in the candidate map. Work from simple to complex: singles first, then subsets and locked candidates, then single-digit patterns, wings, and chains only when simpler scans stop producing progress. Shapedoku uses ordinary classic Sudoku rules rather than Sukaku starting lists, but its Notes make the same candidate discipline easy to practice on Hard and Extreme boards. The symbols have no numerical meaning, so a heart or crystal behaves exactly like a digit identity. If you enjoy puzzles that make candidate structure the main event, Sukaku Sudoku is a focused way to train the skill without guessing.

One common Sukaku Sudoku mistake is treating the printed marks as if they were ordinary notes that can be rebuilt from solved digits. They cannot. The starting candidate list may encode information that is not recoverable from the visible placements alone. If you accidentally erase the wrong mark, use the original puzzle to restore the cell rather than regenerating candidates from scratch. When checking a difficult elimination, ask two questions: was this candidate permitted at the start, and what later deduction removed it? That small audit trail keeps the puzzle logically faithful and makes advanced patterns much easier to review.

• Never add a digit that was absent from the original candidate list for that cell.
• Solve printed single-candidate cells first, then clean their row, column, and box.
• Look for naked and hidden subsets because candidate lists make them easier to expose.
• Scan one digit across the grid when the full pencilmark view feels crowded.
• Use advanced chains only after ordinary singles, subsets, and locked candidates are exhausted.
• Keep every elimination reversible in your reasoning: you should be able to name the rule that removed it.