How to Check if a Sudoku Puzzle is Valid

Описание: Learn how to validate a `Sudoku` solution using Java. Follow our step-by-step guide to ensure your program works correctly without using hashsets.
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This video is based on the question https://stackoverflow.com/q/64595931/ asked by the user 'IIT Topper' ( https://stackoverflow.com/u/13146012/ ) and on the answer https://stackoverflow.com/a/64596226/ provided by the user 'Kerat' ( https://stackoverflow.com/u/7501326/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Check if a Sudoku Puzzle is Valid: A Beginner's Guide

Sudoku is a fascinating number puzzle that challenges our logic and problem-solving skills. However, a common issue many beginners encounter is how to determine if their Sudoku solution is valid. If you’re struggling with your Java program to validate a Sudoku board and it’s returning incorrect results, don’t worry! In this post, we’ll dissect the problem, identify the likely issues, and provide you with a clear solution.

Understanding the Problem

When checking if a Sudoku solution is valid, we must ensure that:

Each number from 1 to 9 appears only once in each row.

Each number appears only once in each column.

Each number appears only once in each of the nine 3x3 subgrids.

The Java code provided has a good structure but it doesn't correctly implement these checks, leading to inaccurate results.

A Breakdown of the Existing Code

Your existing code uses methods to check for the presence of a number in rows, columns, and boxes. However, the current logic checks whether the number exists in those sections without handling the validation appropriately. Here are the methods you had:

containsInRow: Checks if a number exists in the specified row.

containsInCol: Checks if a number exists in the specified column.

containsInBox: Checks if a number exists in the 3x3 box.

The issue arises because you’re performing checks that will always return true since the number is being checked against itself.

The Solution: Steps to Fix the Logic

Step 1: Modify the Checking Functions

Instead of checking for existence, the methods should count occurrences of each number. If a number appears more than once, the Sudoku solution is invalid.

Here’s how to adjust the functions:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Update the isAllowed Method

Now that our checking functions count the occurrences, we can easily validate in the isAllowed method:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Complete the Main Method

Your main method already traverses through the entire board. Just ensure it uses the updated isAllowed method for validation.

Here’s what your main method should look like:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By modifying the containsInRow, containsInCol, and containsInBox methods to count occurrences instead of checking simple existence, you can ensure that your Sudoku validation logic works correctly. This adjustment, combined with the proper validation in the isAllowed method, will help you confirm if a Sudoku solution is valid without resorting to hashsets.

Now, you're set to execute your validation program successfully! Happy coding!