How do I practice coding and algorithm questions?

Use PracHub's coding console to write, test, and debug your solutions in Python or JavaScript. View hints, test against sample inputs, and compare with official solutions.

What difficulty level is this coding question?

This is a medium difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Verkada.

What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Verkada during technical interviews.

Validate a 9×9 grid under constraints | Verkada Coding Question

Quick Overview

This question evaluates the ability to perform constraint validation on a fixed-size 9×9 grid, testing skills in matrix/array handling, duplicate detection and use of simple data structures for correctness checking.

Validate a 9×9 grid under constraints

Company: Verkada

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: medium

Interview Round: Technical Screen

## Problem You are given a 9×9 grid representing a partially filled puzzle. Each cell contains either: - a digit character `'1'`–`'9'`, or - the character `'.'` meaning empty. Determine whether the current filled cells are **valid** under these rules: 1. No digit `'1'`–`'9'` repeats within any row. 2. No digit repeats within any column. 3. No digit repeats within any of the nine 3×3 sub-grids. Return `true` if the filled cells satisfy all rules; otherwise return `false`. ## Input / Output - **Input:** `board` — a 9×9 array of characters - **Output:** boolean ## Notes / Constraints - You only need to validate the currently filled cells; you do **not** need to solve the puzzle. - The board is always 9×9. ## Example If a row contains two `'5'` values (ignoring `'.'`), the board is invalid and you should return `false`.

Quick Answer: This question evaluates the ability to perform constraint validation on a fixed-size 9×9 grid, testing skills in matrix/array handling, duplicate detection and use of simple data structures for correctness checking.

You are given a 9×9 grid representing a partially filled puzzle. Each cell contains either a digit character '1' to '9' or '.' for an empty cell. Determine whether the currently filled cells form a valid state. A state is valid if no digit appears more than once in any row, any column, or any 3×3 sub-grid. You only need to validate the existing filled cells; you do not need to solve the puzzle.

Constraints

board always has exactly 9 rows and 9 columns
Each board[r][c] is either '.' or a character from '1' to '9'
Only the currently filled cells need to be validated

Examples

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']],)

Expected Output: True

Explanation: No filled digit repeats in any row, column, or 3×3 sub-grid.

Input: ([['5','3','.','.','7','.','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']],)

Loading coding console...

Quick Overview

Validate a 9×9 grid under constraints

Constraints

Examples

Hints