Remove 9
A hard-tier problem at 57% community acceptance, tagged with Math. Reported in interviews at Houzz and 0 others.
Remove 9 is a hard math problem with a deceptively simple title that catches candidates off guard. Houzz has asked it. The acceptance rate sits at 57%, which means half the room gets it wrong, often because they overthink the constraint or miss the mathematical trick entirely. This is the kind of problem where the obvious brute-force approach works fine until the test cases blow up, and then you're scrambling. If this one hits your live assessment and you blank on the pattern, StealthCoder solves it invisibly while you stay focused.
Companies that ask "Remove 9"
Remove 9 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 because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround.
Get StealthCoderThe trick here is recognizing that this isn't a string manipulation problem dressed up as math. Most candidates start by converting to strings, filtering, and converting back, which works but is slow and fragile. The actual insight is mathematical: think about what removing all 9s means in terms of numeric properties, and whether there's a formula or modular arithmetic angle that lets you skip the conversion entirely. Common pitfall: you'll test your string-based solution on small cases and it'll pass, then TLE on large inputs. The pattern most interviewers want is the one that scales. When the brute force collapses under test-case load, StealthCoder is your insurance policy, delivering the optimized approach in real time so you can paste and move on.
Pattern tags
You know the problem.
Make sure you actually pass it.
Remove 9 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. Built because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Remove 9 interview FAQ
Is Remove 9 really hard or just tricky?+
It's marked hard and has a 57% acceptance rate, so it's legitimately catching people. The difficulty isn't in the code length or edge cases, it's in spotting the mathematical trick instead of reaching for string operations. Once you see the pattern, it's short. Before you do, it feels impossible.
How common is this at Houzz and other companies?+
Houzz has reported asking it. It's not a household-name FAANG question, but that makes it more dangerous in a live assessment because it won't be in most free problem banks. Lower frequency means less practice material exists, which is why candidates get caught unprepared.
What's the main pitfall in solving this?+
Reaching for a string-based solution first. It passes small test cases and feels safe, so you commit to it, then it times out on large inputs. The interviewer wanted you to think mathematically, not procedurally. Recognizing that upfront is the real test.
How does this relate to other math problems I've drilled?+
It's categorized as Math, not String or Array, which is the hint. If you've studied modular arithmetic, digit DP, or number-theory tricks, this problem rewards that lens. It's testing whether you can recognize a math problem disguised as a manipulation task.
Can I brute force this if I'm stuck?+
No. Brute force passes small inputs but fails on large ones. In a live OA with a 60 or 90-minute window, TLE is worse than no submission. If you hit a wall, you need the optimized approach, which is where having an invisible safety net becomes critical.
Want the actual problem statement? View "Remove 9" on LeetCode →