Check Whether a Partially Filled 9x9 Sudoku Board Is Valid
Company: Confluent
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Onsite
You are given a `9 x 9` Sudoku board in which some cells are filled with digits and the rest are empty. Determine whether the filled cells are consistent with the rules of Sudoku.
### Function Signature
```python
def is_valid_board(board: list[list[str]]) -> bool:
```
### Rules
- Return `True` if and only if all three conditions hold:
- no row contains the same digit twice;
- no column contains the same digit twice;
- none of the nine `3 x 3` boxes contains the same digit twice. The boxes are the blocks formed by rows `0-2`, `3-5`, `6-8` crossed with columns `0-2`, `3-5`, `6-8`.
- Only filled cells are checked. Empty cells never cause a violation.
- Do not check whether the board can be completed. A board that satisfies the three conditions is valid even if no full solution extends it.
### Constraints
- `len(board) == 9` and `len(board[i]) == 9` for every row `i`.
- Every cell is one of the digit characters `"1"` through `"9"`, or `"."` for an empty cell.
### Examples
**Example 1**
```text
Input: board = [
["2", ".", ".", "5", ".", ".", "1", ".", "."],
[".", "3", ".", ".", "9", ".", ".", "6", "."],
[".", ".", "7", ".", ".", "4", ".", ".", "8"],
["9", ".", ".", "6", ".", ".", "3", ".", "."],
[".", "8", ".", ".", "7", ".", ".", "4", "."],
[".", ".", "5", ".", ".", "1", ".", ".", "2"],
["4", ".", ".", "8", ".", ".", "7", ".", "."],
[".", "1", ".", ".", "2", ".", ".", "5", "."],
[".", ".", "6", ".", ".", "3", ".", ".", "9"]
]
Output: True
```
No row, column or box repeats a digit among its filled cells.
**Example 2**
```text
Input: board = [
["2", ".", ".", "5", ".", ".", "1", ".", "."],
["7", "3", ".", ".", "9", ".", ".", "6", "."],
[".", ".", "7", ".", ".", "4", ".", ".", "8"],
["9", ".", ".", "6", ".", ".", "3", ".", "."],
[".", "8", ".", ".", "7", ".", ".", "4", "."],
[".", ".", "5", ".", ".", "1", ".", ".", "2"],
["4", ".", ".", "8", ".", ".", "7", ".", "."],
[".", "1", ".", ".", "2", ".", ".", "5", "."],
[".", ".", "6", ".", ".", "3", ".", ".", "9"]
]
Output: False
```
The top-left box holds `7` at `(1, 0)` and at `(2, 2)`. No row or column repeats a digit, so the box rule alone makes the board invalid.
Overview: A coding problem that asks whether a partially filled 9x9 Sudoku board is consistent, meaning no row, column or 3x3 box repeats a digit among its filled cells. It tests careful bookkeeping of row, column and box membership without attempting to solve the puzzle.
Read the full Confluent Software Engineer interview experience this question came from