---
title: '36. Valid Sudoku'
description: 'Determine if a 9 x 9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:'
icon: dot
topics:
  - { name: "Array", slug: "array" }
  - { name: "Hash Table", slug: "hash-table" }
  - { name: "Matrix", slug: "matrix" }
issue: "https://github.com/prdlk/leetcode/issues/94"
companies:
  - { name: "Amazon", period: "Thirty Days", frequency: 48.9 }
  - { name: "Apple", period: "Three Months", frequency: 69 }
  - { name: "Bloomberg", period: "Three Months", frequency: 36.8 }
  - { name: "Google", period: "Three Months", frequency: 23.9 }
  - { name: "Confluent", period: "Six Months", frequency: 100 }
  - { name: "Meta", period: "Six Months", frequency: 32.7 }
  - { name: "Microsoft", period: "Six Months", frequency: 32 }
sidebar:
  label: 'Valid Sudoku'
  badge: 'Medium'
---

### 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:

- `board.length == 9`
- `board[i].length == 9`
- board[i][j] is a digit 1-9 or '.'.

## Solution

```py
import collections


class Solution:
    def isValidSudoku(self, board: list[list[str]]) -> bool:
        cols = collections.defaultdict(set)
        rows = collections.defaultdict(set)
        squares = collections.defaultdict(set)  # (r/3, c/3)

        for r in range(9):
            for c in range(9):
                if board[r][c] == ".":
                    continue
                if (
                    board[r][c] in rows[r]
                    or board[r][c] in cols[c]
                    or board[r][c] in squares[(r // 3, c // 3)]
                ):
                    return False
                cols[c].add(board[r][c])
                rows[r].add(board[r][c])
                squares[(r // 3, c // 3)].add(board[r][c])

        return True
```