A Sudoku Solver Step-by-Step is an interactive tool that solves Sudoku puzzles one logical deduction at a time. Instead of instantly filling all cells, it explains each reasoning step, showing which strategy was used (e.g., Naked Single, Hidden Single), which cell was affected, and why the number must go there. This makes it perfect for learning Sudoku solving techniques and understanding the logic behind each move.

This tool implements several fundamental strategies:

• Naked Single: A cell has only one possible candidate number after eliminating numbers already present in its row, column, and 3×3 box.
• Hidden Single: A number can only go in one cell within a row, column, or 3×3 box, even if that cell has other candidates.
• Candidate Elimination: Automatically removes impossible candidates based on existing numbers in the same row, column, and box.

More advanced strategies may be added in future updates.

A Naked Single occurs when a cell has exactly one candidate number remaining after eliminating all numbers that already appear in the same row, column, or 3×3 box. For example, if a cell can only be a 7 (because 1,2,3,4,5,6,8,9 are all ruled out), it's a Naked Single. This is the most straightforward solving technique and often the first one beginners learn.

A Hidden Single occurs when a specific number (say, 5) can only go in one cell within a particular row, column, or 3×3 box—even though that cell may have multiple candidates. The number is "hidden" among other candidates. For instance, if 5 can only be placed in one cell of a row (because all other cells either already contain numbers or cannot be 5), that cell must be 5, regardless of what other candidates it has.

1. Load a puzzle: Choose an example from the dropdown menu (Easy, Medium, or Hard), or manually click cells and type numbers 1-9 to enter your own puzzle.
2. View candidates: Toggle the "Show Candidate Numbers" switch to see pencil marks (possible numbers) in each empty cell.
3. Step through: Click Next Step to execute one logical deduction at a time. The relevant cells will be highlighted, and an explanation appears in the strategy panel.
4. Auto Solve: Click Auto Solve to run through all remaining steps automatically with a short delay between each.
5. Undo: Revert the last solver step if you want to review it again.
6. Reset: Clear all solved cells and return to the original puzzle.

This solver can handle most easy and medium-difficulty puzzles using Naked Single and Hidden Single strategies. For harder puzzles that require advanced techniques (such as X-Wing, Swordfish, XY-Wing, or forcing chains), the solver may get stuck. When this happens, it will notify you that no more logical steps can be found with the current strategy set. We recommend using this tool primarily for learning and practicing fundamental Sudoku techniques.

The color coding helps you understand the solver's reasoning:

• Yellow highlight: The cell being solved in the current step.
• Light blue/purple: Cells related to the current deduction (same row, column, or box).
• Blue number: A number just filled in by the solver.
• Dark number: An originally given number.
• Gray background: Cells containing given (original) numbers.
• Orange candidate number: The key candidate being highlighted in the current step.

Candidates (also called pencil marks) are the possible numbers that could go in an empty cell, after eliminating numbers that already appear in the same row, column, and 3×3 box. For example, if a cell's row contains 1, 3, and 7; its column contains 2 and 8; and its box contains 4 and 6—then the candidates for that cell are {5, 9}. Displaying candidates is essential for solving harder puzzles and is a standard technique used by Sudoku players worldwide.