Find the Length of the Longest Common Prefix
A medium-tier problem at 56% community acceptance, tagged with Array, Hash Table, String. Reported in interviews at The Trade Desk and 6 others.
You're staring at an array of strings and the problem asks for the longest common prefix. Sounds simple until you realize the brute force horizontal scan is slow, and the vertical scan leaves edge cases wide open. This problem appears regularly at The Trade Desk, ZipRecruiter, Capital One, Coinbase, Roblox, Visa, and Databricks. The acceptance rate sits at 56%, which means half the candidates either time out or miss the optimization. If you blank on the trick during your live assessment, StealthCoder runs invisibly and surfaces a working solution in seconds.
Companies that ask "Find the Length of the Longest Common Prefix"
Find the Length of the Longest Common Prefix 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 Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE.
Get StealthCoderThe gotcha is that most people scan left to right (horizontal), comparing all strings at position 0, then position 1, and so on. That works but it's verbose and easy to break on empty arrays or single-string inputs. The smarter move is a Trie or a binary search on prefix length, cutting the time complexity significantly. Common mistakes: forgetting to handle empty input, assuming all strings are non-empty, or overcomplicating the string comparison logic. The problem tests whether you can optimize beyond the obvious iteration. This lands in the medium bucket because it rewards knowing the pattern but punishes the person who defaults to horizontal scan without thinking. If this problem hits your live OA and you're unsure about the optimal structure, StealthCoder solves it in seconds, invisible to the proctor.
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Find the Length of the Longest Common Prefix 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. Made by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Find the Length of the Longest Common Prefix interview FAQ
Is the brute force horizontal scan enough to pass?+
It'll pass most test cases, but with 56% acceptance and 7 major companies asking it, the hidden cases probably include large arrays or very long strings. You'll likely hit a time limit or run out of confident edge case handling. The Trie or binary search approach is safer.
How does this relate to the Trie topic?+
A Trie lets you build the common prefix structure incrementally. Insert all strings into a Trie and walk down the branches where each node has exactly one child. That's your common prefix. It's O(n*m) but clean and hard to mess up.
What's the trick to avoid timeout?+
Binary search on the prefix length (1 to min string length), then check if that prefix is common to all strings. Or use a Trie and stop early. Both beat horizontal scan on large inputs. The problem rewards knowing one of these two patterns.
Does this problem appear frequently at FAANG?+
Not FAANG proper, but it shows up at solid companies like Coinbase, Visa, and Capital One. The 56% acceptance and 7-company report suggest it's stable in mid-tier and fintech hiring loops. Not rare, not super common.
What's the most common mistake?+
Assuming the first string is always a valid prefix and comparing all others to it, then failing on empty input or single-element arrays. Also, off-by-one errors in the loop bounds. Test empty array, single string, and fully disjoint prefixes first.
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