Reference: LeetCode
Difficulty: Medium

## Problem

Determine if a 9x9 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 9 3x3 sub-boxes of the grid must contain the digits 1-9 without repetition.

The Sudoku board could be partially filled, where empty cells are filled with the character ‘.’.

Example:

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.
• The given board contain only digits 1-9 and the character '.'.
• The given board size is always 9x9.

## Analysis

### Hash Set

Check three cases respectively.

Time: $O(1)$
Space: $O(1)$

Or, we can use 27 hash set to achieve one-pass solution.

Comment
Junhao Wang
Hi, I was a master student at USC studying Computer Science.