Valid Sudoku
A medium-tier problem at 62% community acceptance, tagged with Array, Hash Table, Matrix. Reported in interviews at Riot Games and 31 others.
Valid Sudoku shows up at Uber, Apple, MongoDB, Snap, and a dozen other companies every hiring cycle. It looks simple: check if a partially-filled 9x9 board is valid. The catch is efficiency. Candidates who try nested loops and redundant checks hit O(n^2) or worse. Others misunderstand what "valid" means and waste time on incomplete boards. The acceptance rate sits at 62%, which tells you plenty of people stumble here. StealthCoder is your safety net if you blank on the hash table pattern during the live assessment.
Companies that ask "Valid Sudoku"
Valid Sudoku is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Made by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE.
Get StealthCoderThe trick is single-pass validation using three hash tables: one for rows, one for columns, one for 3x3 boxes. Most candidates default to nested loops and waste cycles. The pattern becomes obvious once you see it: iterate the board once, and for each non-empty cell, check if the digit exists in its row, column, and box hash set. If it does, return false. If you finish the loop, return true. This is O(81) = O(1) time and space, since the board is always 9x9. Common fail is indexing the 3x3 box incorrectly (use (row/3)*3 + col/3). StealthCoder surfaces the exact hash table setup and iteration pattern in seconds if you hit this during a live online assessment.
Pattern tags
You know the problem.
Make sure you actually pass it.
Valid Sudoku recycles across companies for a reason. It's medium-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Made by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Valid Sudoku interview FAQ
Is Valid Sudoku actually asked at FAANG?+
Yes. Apple, MongoDB, Uber, and others ask it regularly. 62% acceptance rate means it's not a filtering problem, but you're expected to nail the hash table approach cleanly. Missing it signals you don't know the standard pattern.
What's the trick to Valid Sudoku?+
One pass, three hash tables. Row hash, column hash, 3x3 box hash. For each cell, check membership in all three before inserting. The box index formula is (row/3)*3 + col/3. Miss that formula and you'll fail edge cases.
How does this relate to the Matrix topic?+
The sudoku board is a 2D matrix. You iterate it row by row, but you also track constraints across rows and columns simultaneously. Thinking in terms of matrix indices and box boundaries is the core skill.
What if I freeze on the hash table approach during the OA?+
StealthCoder reads the problem and serves a working solution in seconds, invisible to the proctor. You copy the pattern, paste it into the editor, and move on. No one knows you used it.
Is there a tricky edge case?+
The 3x3 box indexing trips people. Box number is (row/3)*3 + col/3, using integer division. Also, empty cells (dots) don't count as violations. Most failures come from wrong box logic, not the row/column hash logic.
Want the actual problem statement? View "Valid Sudoku" on LeetCode →