PuzzleBirch · Solver
Sudoku solver with hints
Type in the puzzle you are stuck on. Ask for one hint at a time, with the technique named and explained, or solve the whole grid at once.
Click a square or use the arrow keys, then type 1 to 9. Delete, Backspace or 0 clears a square.
Press Hint for the next step a person could take, or Solve to fill in everything. Change any square and the help starts again from there.
Difficulty: Medium. 26 digits given.
How the solver and hints work
Enter the puzzle by clicking a square and typing, or paste all 81 squares at once: digits for the givens and 0, a dot or a dash for the blanks. Grids copied from books and websites usually paste straight in, lines and all.
Hint looks for the next digit a person could find by reasoning, never by guessing. It starts with the simplest ideas: a hidden single (in a row, column or box, a digit has only one square left) and a naked single (a square has only one digit left). When those run out it tries pointing pairs, naked and hidden pairs and triples, X-wings, XY-wings and a few more, and tells you each step it took. The new digit appears in red, with its row, column or box shaded.
Solve fills in the rest with a fast search, and Check tells you whether the digits so far can still lead to a solution. Mistakes are caught early: a digit repeated in a row, column or box turns red, and a grid with no solution, or with more than one, says so before any hint is given.
Questions
Can it solve every sudoku?
Yes. Solve uses a search that finishes even the hardest known puzzles in a fraction of a second. Hints are limited to techniques a person would use, so on extremely hard grids they can run out; the solver then says so and offers to reveal a square.
What does “more than one solution” mean?
A proper sudoku has exactly one answer. If a given digit was missed or mistyped, several grids can fit, and no amount of logic can choose between them. Check the puzzle against the original.
How is the difficulty worked out?
By the hardest technique needed when always taking the simplest step available: singles only is Easy, pairs and pointing is Medium, X-wings and XY-wings are Hard, and chains such as the skyscraper are Expert.