LeetCode 36 ·
Medium
Valid Sudoku
Determine whether a partially filled 9×9 board breaks any Sudoku rule.
Try it
Step through the core mechanic. The simulator below runs the hash map shape this problem is built on.
Walk the pattern
No dedicated step-through for this one yet. The shape is Hash map — its pattern page has the interactive walkthrough, the reference implementation, and a five-problem progression that this problem sits inside.
The approach
Keep a seen-set per row, per column, and per 3×3 box (box index = (r/3)·3 + c/3). One pass; a duplicate in any of the three sets fails.
| Aspect | Value |
|---|---|
| Pattern | Hash map |
| Recognise it by | Constraint check across rows, columns, boxes. |
| Time complexity | O(1) |
| Space complexity | O(1) |
| Difficulty | Medium |
Who asks it
Companies known to ask this problem, from public LeetCode company-tag aggregations. A signal of where to expect it, not a guarantee.