Check If Word Is Valid After Substitutions
A medium-tier problem at 60% community acceptance, tagged with String, Stack. Reported in interviews at Nutanix and 0 others.
You're staring at a string that can be recursively rewritten. Nutanix has asked this one. The problem looks deceptive: you need to detect whether a word can be reduced to nothing by replacing substrings with nothing, following specific rules. Most candidates jump straight to brute-force replacements and time out, or they get tripped up by overlapping matches. The actual pattern is a single-pass stack solution. If this lands in your live OA and you freeze on the approach, StealthCoder surfaces the stack trick in seconds, invisible to the proctor.
Companies that ask "Check If Word Is Valid After Substitutions"
Check If Word Is Valid After Substitutions 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 at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share.
Get StealthCoderThe key insight is that you don't need to repeatedly scan and replace. Build a stack and process the string character by character. When the top of the stack plus the current character matches a forbidden substring, pop instead of push. If you don't recognize this pattern upfront, you'll waste time on nested loops or regex replacements that collapse under larger test cases. The Stack and String topics are essential here; this is a stack problem disguised as a string problem. StealthCoder is your hedge if the pattern doesn't click during the assessment and you're running low on time.
Pattern tags
You know the problem.
Make sure you actually pass it.
Check If Word Is Valid After Substitutions recycles across companies for a reason. It's medium-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 at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Check If Word Is Valid After Substitutions interview FAQ
How often does Nutanix ask this problem?+
Nutanix is the only company in current reports asking this specific problem. That doesn't mean it won't appear elsewhere, but the low company count means it's a niche ask. Prepare it, but don't obsess if you haven't drilled it before the OA.
What's the trick I'm missing if I try nested loops?+
Nested loops with repeated string replacements have worst-case exponential time. The stack approach solves it in a single pass. Build the stack, check the last few characters on each push, and pop on match. No backtracking, no re-scanning the whole string.
Is this really a medium-difficulty problem?+
The 60% acceptance rate suggests it's medium-hard in practice. The trick is non-obvious if you haven't seen stack-based substring matching before. Once you know the pattern, it's straightforward implementation.
Does this require knowing regex or advanced string methods?+
No. Plain string indexing and a list or array as a stack are all you need. The challenge is the algorithmic insight, not the language features. Any language with basic data structures will work.
How does this relate to other stack problems I should know?+
This is a variant of parentheses matching and string validation using stacks. If you're solid on valid parentheses and remove duplicates from a string, the core technique carries over. Stack problems often hide as string problems.
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