EvilSudoku.Online

Difficulty60

Mistakes0

Time00:00

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    What is Sudoku?

    Sudoku is a Japanese logical puzzle invented in 1980s which grew to being one of the most popular puzzle games of all time. It is widely appealing because it requires logical reasoning without any math skill. It has a clear and simple set of rules which can produce puzzles of various complexities.

    Classic Sudoku is played on a 9×9 grid, divided into nine 3×3 subgrids (often called “boxes” or “blocks”). The objective of the puzzle is to fill the grid so that every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once. Some of the numbers are given at the start of the game (called clues or givens). The rest must be filled in using logical reasoning techniques—deduction patterns used to eliminate candidate digits and determine digit placement without guessing.

    A well-structured Sudoku puzzle has exactly one valid (“unique”) solution that satisfies all row, column, and 3×3 box constraints while respecting the given clues. This means that each empty cell accepts only one specific digit as a valid solution. For a given puzzle, there may be multiple logical reasoning paths (the order in which the cells are solved) leading to the same (“unique”) solution of the whole grid.

    How to play Sudoku?

    All Sudoku techniques (also called “strategies”) are based on only 3 principles:

    1. If a row already has digit X, digit X will not be present in the same row again.
    2. If a column already has digit X, digit X will not be present in the same column again.
    3. If a box already has digit X, digit X will not be present in the same box again.

    The easiest Sudoku puzzles can be solved using just two most fundamental techniques:

    • Naked Singles: If a row, column, or box has only one empty cell, figure out which digit is missing and place it in the empty cell. The easiest method for getting to know which digit is missing is trying to find each digit starting from “1”. The first digit that can't be found should be placed in the empty cell. Advanced Sudoku players are able to visually scan whole rows (or columns or boxes) and identify which digits are missing in a split second.
    • Hidden Singles: If combined constraints of a row and a column and a box allow for only one digit to appear in a cell, place the only remaining digit in the cell. For example, if the row of the cell X is missing only digits “1”, “2” and “3” while the column of the cell X is missing “2”, “3” and “4” and the box of the cell X is missing only “3” and “5”, then the combined constraints for cell X allow for only digit “3” to be placed in it, as it is the only digit missing in the row, the column and the box for this cell.

    For detailed descriptions of many more Sudoku techniques, please visit Sudoku Rules page.

    What is Evil Sudoku?

    Evil Sudoku is an informal difficulty classification for a standard 9×9 Sudoku puzzle that is at (or near) the highest level of logical complexity. The rules are identical to classic Sudoku. The difference lies purely in logical depth required to solve it. Evil Sudokus are designed to suppress logical elimination, require deep pattern recognition, and punish careless assumptions.

    An “Evil” puzzle typically has very sparse given numbers—only 17–23 initial clues. However, clue count alone does not determine difficulty. Early-stage solving yields very few naked singles or other obvious placements. Solving usually requires multiple higher-order techniques such as Hidden Pairs, Hidden Triples, Pointing Pairs, Pointing Triples, X-Wing, Y-Wing, and Swordfish.