Modify the Matrix
A easy-tier problem at 68% community acceptance, tagged with Array, Matrix. Reported in interviews at Fidelity and 0 others.
Modify the Matrix is an easy array problem that shows up in Fidelity interviews. It's the kind of problem that looks trivial at first glance, then catches candidates off guard when they misread the constraint or implement the naive approach without thinking. With a 68% acceptance rate, it's designed to separate people who code before they think from people who actually parse the problem. If this one lands in your live assessment and you blank on the pattern, StealthCoder surfaces a working solution in seconds while you stay invisible to the proctor.
Companies that ask "Modify the Matrix"
Modify the Matrix 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. Built by an engineer who got tired of watching his cohort grind for six months and still get filtered at the OA stage.
Get StealthCoderThe trick here is recognizing that you can't modify the matrix in place the way your first instinct suggests. Candidates typically try to update values as they scan the grid, then realize they've corrupted data they still need to read. The correct approach is to separate the read phase from the write phase, or mark cells for modification without changing them until you've scanned the full matrix. It's not hard once you see it, but the problem statement is designed to make you write code before you've thought through what 'modify' actually means in context. Common pitfall: overwriting a cell and then needing its original value three rows down. StealthCoder handles the real-time pattern recognition during your assessment if you hit that wall.
Pattern tags
You know the problem.
Make sure you actually pass it.
Modify the Matrix recycles across companies for a reason. It's easy-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. Built by an engineer who got tired of watching his cohort grind for six months and still get filtered at the OA stage. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Modify the Matrix interview FAQ
Is Modify the Matrix actually easy, or is the difficulty rating inflated?+
The 68% acceptance rate confirms it's genuinely easy once you understand the constraint. The catch is parsing the problem correctly before coding. If you read carefully and trace through an example, the solution becomes obvious. Most rejections come from implementing without thinking first, not from the algorithmic complexity.
What's the core trick that makes this problem work?+
You need to identify that you can't modify values in place as you read them. Either use a separate data structure to mark changes, or scan the entire matrix first to collect all modifications, then apply them in a second pass. The problem tests whether you think through side effects before coding.
Do I need to optimize for space, or is any O(n) extra space solution acceptable?+
The input data doesn't specify a strict space constraint, and at the easy level, Fidelity is testing pattern recognition, not optimization. A clean two-pass or marked-cell solution will pass. Don't over-engineer at the cost of clarity.
How does this connect to other array and matrix problems I should know?+
Modify the Matrix shares DNA with problems that test read-modify-write logic, like Set Matrix Zeroes. Both require you to recognize that modifying in place corrupts data you still need. Understanding this pattern transfers directly to other grid-manipulation problems.
Should I expect Modify the Matrix to come up in other company assessments, or is it Fidelity-specific?+
The data shows only Fidelity has reported this problem. That said, the underlying pattern of safe in-place modification is common across technical screens. If you understand the solution here, you're prepared for similar problems elsewhere.
Want the actual problem statement? View "Modify the Matrix" on LeetCode →