Min Num Cells
Reported by candidates from Microsoft's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
You've got a Microsoft OA incoming in the next 48 hours and "Min Num Cells" is sitting in your problem list. No problem text provided yet, but the title alone tells you this is a grid or matrix traversal question where you need to find the minimum number of cells required to satisfy some constraint. Microsoft loves these because they test both pathfinding and optimization instincts. If you blank on the live OA, StealthCoder will read the exact problem and feed you the pattern in real time.
Pattern and pitfall
The core mechanic here is almost always dynamic programming, BFS, or greedy selection on a grid. You're minimizing a cell count, which means you need to either find the shortest path that covers certain cells, the fewest cells needed to block/reach a target, or cells required to satisfy a coverage rule. The trick Microsoft uses: the naive greedy approach (pick the cell that covers the most) doesn't always work if there are overlapping constraints or ordering dependencies. Map out the grid structure first, identify what "minimum" means in context, then decide if you're doing DP (state is position and coverage), BFS (each state is a cell count), or a weighted selection. StealthCoder will see the problem statement and confirm which one fits.
Drill it cold or hedge it with StealthCoder. Either way, don't walk into the OA hoping you remember the trick.
You can drill Min Num Cells cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Made for the candidate who got the OA invite this morning and has 72 hours, not six months.
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Microsoft reuses patterns across OAs. Made for the candidate who got the OA invite this morning and has 72 hours, not six months. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Min Num Cells FAQ
Is this a grid DP or BFS problem?+
Without the full problem text it's hard to call, but Microsoft usually frames these as "find minimum cells to reach X" or "cover region Y with fewest cells." If order matters or states overlap, it's DP. If you're exploring outward level by level, it's BFS. The OA will be clearer when you see it.
What's the common trap with min cell problems?+
Greedy fails. You can't just always pick the cell that covers the most ground. You might need to pick a suboptimal cell now to unlock a better path later. That's why you often need DP or full search with memoization.
How do I solve this in under 25 minutes?+
Parse the grid and the constraint first. Write a recursive or DP solution with memoization. Test on the example they give. Optimize only if it times out. Microsoft rarely asks for complex pruning tricks on these, so a clean DP solution usually passes.
Is this still asked at Microsoft in mid-2024?+
Yes. Grid optimization and pathfinding are bread and butter for Microsoft backend and cloud roles. The specific framing changes, but the pattern stays. Expect this class of problem.
What if I completely blank on the live OA?+
That's exactly why you'd use StealthCoder. It reads the actual problem statement, identifies the pattern, and shows you the solution structure before you write a line. You go from panic to confidence in seconds.