Stamping the Grid
A hard-tier problem at 33% community acceptance, tagged with Array, Greedy, Matrix. Reported in interviews at Rubrik and 0 others.
Stamping the Grid is a hard matrix problem that hits you with a greedy twist. It's asked at Rubrik and sits at a 33% acceptance rate, which means most people who see it live get stuck on how to structure the stamp-placement logic. The trap is obvious: iterate through positions and try to place stamps. The real problem is figuring out which placement order actually matters and how to avoid overwriting. If you blank on the greedy strategy during your OA, StealthCoder runs invisibly and surfaces the working approach before you spiral.
Companies that ask "Stamping the Grid"
Stamping the Grid 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 FAANG engineer who treats the OA the way companies treat hiring: as a game with rules you should know.
Get StealthCoderThe core trick here is recognizing that you need to work backwards or use a greedy heuristic to place stamps on a matrix without conflicts. The naive approach of just placing stamps left-to-right fails because later stamps can block valid placements. You need to layer in prefix sums to quickly validate whether a region is all zeros or already stamped. The hard part isn't the array iteration, it's the insight that greedy works and what order to stamp in. Most candidates waste 20 minutes trying a recursive backtracking path before realizing greedy + prefix sum is the actual pattern. That's exactly where StealthCoder saves you during a live assessment, cutting through the false starts.
Pattern tags
You know the problem.
Make sure you actually pass it.
Stamping the Grid recycles across companies for a reason. It's hard-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 FAANG engineer who treats the OA the way companies treat hiring: as a game with rules you should know. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Stamping the Grid interview FAQ
Why is Stamping the Grid only 33% acceptance?+
Because the greedy insight isn't obvious and most candidates try backtracking or brute-force validation first. The trick is recognizing you can work backwards or use a smart placement order combined with prefix sums. Without that pattern, the problem feels unsolvable in the time window.
Is this more about matrix knowledge or greedy intuition?+
It's greedy intuition with matrix mechanics. You need to understand how stamps overlap and conflict, but the core insight is that not all placement orders are equal. Prefix sum helps you validate efficiently, but greedy is the conceptual unlock.
Does Rubrik ask this often?+
It's reported from one major company in this data set, so treat it as a differentiated hard problem. That low frequency makes it a wild card. If it hits your OA, you're not expected to have drilled it a dozen times like easier problems.
How do I know when to use prefix sum here vs. brute force validation?+
Brute force validation of each region is O(n*m) per stamp and will TLE. Prefix sum lets you check any region in O(1) after O(n*m) preprocessing. The problem scale almost always forces the prefix sum path once you recognize regions need repeated validation.
Should I practice greedy matrix problems before tackling this?+
It helps to know how greedy works on grids and how prefix sums accelerate range queries, but this problem is hard enough that a cold read is rough. If you've drilled prefix sum basics and greedy on simpler arrays, you're roughly ready. This is the point where pattern knowledge makes the difference.
Want the actual problem statement? View "Stamping the Grid" on LeetCode →