Valid Sudoku
Determine if a 9 × 9 Sudoku board is valid.
Only the filled cells need to be validated according to the following rules:
1. Each row must contain the digits 1-9 without repetition.
2. Each column must contain the digits 1-9 without repetition.
3. Each of the nine 3 × 3 sub-boxes must contain the digits 1-9 without repetition.
Note:
A Sudoku board may be valid but is not necessarily solvable.
Empty cells are represented by '.'.
Example 1
Input
5 3 . . 7 . . . . 6 . . 1 9 5 . . . . 9 8 . . . . 6 . 8 . . . 6 . . . 3 4 . . 8 . 3 . . 1 7 . . . 2 . . . 6 . 6 . . . . 2 8 . . . . 4 1 9 . . 5 . . . . 8 . . 7 9
Output
true
Explanation
Every row, column, and 3 × 3 sub-box satisfies the Sudoku rules.
Example 2
Input
8 3 . . 7 . . . . 6 . . 1 9 5 . . . . 9 8 . . . . 6 . 8 . . . 6 . . . 3 4 . . 8 . 3 . . 1 7 . . . 2 . . . 6 . 6 . . . . 2 8 . . . . 4 1 9 . . 5 . . . . 8 . . 7 9
Output
false
Explanation
The digit '8' appears twice in the first column, violating Sudoku rules.
Constraints
Hints:
Hint 1
Check each row, each column, and each 3 × 3 sub-box independently.
Hint 2
Use three hash sets (or boolean arrays) to track digits seen in rows, columns, and sub-boxes. If a digit is encountered twice in the same row, column, or box, the board is invalid.
Author & Technical Reviewer
Technically reviewed by: ExamAdda Technical Review Team
Technical Reviewers, ExamAdda
Software engineers at ExamAdda who check every article's definitions, complexity claims and code examples before and after publishing.
Published
Aug 6, 2026
Last updated
Aug 7, 2026
Content Verification Methodology
Definitions and complexity claims were checked against authoritative computer-science references. Code examples were compiled and tested with standard, boundary and edge-case inputs.
Expected Output