Is Valid Sudoku easy or hard?

Valid Sudoku is generally categorized as a medium difficulty problem. The logic is straightforward once you separate row, column, and subgrid validation, but implementing the checks cleanly requires careful indexing and data structure selection.

Valid Sudoku Python/Java solution

Python and Java solutions typically use three arrays of sets: one for rows, one for columns, and one for 3x3 boxes. During iteration, each digit is validated against these structures before insertion. The algorithm runs in O(n^2) time and is easy to implement in most languages.

How to solve Valid Sudoku in O(n)?

The board must be checked cell by cell, so the practical complexity is O(n^2) for an n×n grid. However, each validation operation is constant time using hash sets or bit masks. This keeps the solution efficient even during full board traversal.

What is the best approach for Valid Sudoku?

The hash set validation approach is the most common solution. Track digits seen in each row, column, and 3x3 subgrid using sets and detect duplicates in O(1) time. The full scan of the board takes O(n^2) time for a 9x9 grid. Bit manipulation is an optimized alternative with constant space.

Is Valid Sudoku asked at Google/Amazon/Meta?

Valid Sudoku appears frequently in coding interviews and practice sets used by companies like Amazon, Google, and Meta. It tests array traversal, constraint validation, and efficient use of hash structures or bit manipulation.

What data structure is used in Valid Sudoku?

The most common data structures are hash sets or hash maps to track digits seen in rows, columns, and boxes. An alternative implementation uses integer bitmasks to store digit presence more compactly while maintaining constant-time checks.

What is the time complexity of Valid Sudoku?

Valid Sudoku runs in O(n^2) time because every cell in the board must be inspected once. For the standard 9x9 board this equals 81 checks, effectively constant time. Each validation step performs O(1) lookups using hash sets or bit masks.

Description Solution Video 13:28

36. Valid Sudoku

Medium64.9% acceptance26 companies

Determine if a 9 x 9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:

Each row must contain the digits 1-9 without repetition.
Each column must contain the digits 1-9 without repetition.
Each of the nine 3 x 3 sub-boxes of the grid must contain the digits 1-9 without repetition.

Note:

A Sudoku board (partially filled) could be valid but is not necessarily solvable.
Only the filled cells need to be validated according to the mentioned rules.

Example 1:

Input: board = 
[["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

Example 2:

Input: board = 
[["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: Same as Example 1, except with the 5 in the top left corner being modified to 8. Since there are two 8's in the top left 3x3 sub-box, it is invalid.

Constraints: