Quora·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Interviewed for a software engineer role at Quora, got a pretty standard string manipulation problem. Nothing too wild but I definitely overthought parts of it.

Questions Asked (1)

Given an array of strings, write a function that returns the longest common prefix shared by all of them. Return an empty string if none exists.

Algorithms & Data Structures

Author's notes

I knew this one but still fumbled the explanation a bit.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying edge cases (empty array, single string, empty strings) and then propose a vertical scanning approach: compare characters column by column across all strings until a mismatch is found. This is efficient and easy to reason about, with O(N*M) time and O(1) space where N is the number of strings and M is the length of the shortest string.

Pro tip: Mention that you can optimize by first finding the shortest string to limit comparisons, and discuss trade-offs with other methods like divide-and-conquer or binary search. Also, explicitly state that you would test with edge cases like empty input, single string, and strings with no common prefix.

1. Clarify requirements and edge cases

Ask about input constraints: can the array be empty? Can strings be empty? What should be returned in those cases? Confirm that the function should return an empty string if no common prefix exists.

2. Choose an approach

Select a strategy such as vertical scanning, horizontal scanning, divide-and-conquer, or binary search. Explain why vertical scanning is often preferred for its simplicity and efficiency.

3. Walk through the algorithm

Describe the step-by-step process: iterate over characters of the first string (or shortest string), and for each character, check if all other strings have the same character at that position. Stop at the first mismatch or when a string ends.

4. Analyze complexity

State the time complexity: O(N * M) where N is the number of strings and M is the length of the shortest string. Space complexity is O(1) extra space (or O(M) if building the result string).

5. Test with examples

Walk through a few test cases: e.g., ['flower','flow','flight'] returns 'fl'; ['dog','racecar','car'] returns ''; empty array returns ''; single string returns itself.

Key Points to Mention

Edge cases: empty array, single string, empty strings, no common prefix.
Vertical scanning vs. horizontal scanning: trade-offs and why vertical is often better.
Time and space complexity analysis.
Optimization: limit comparisons to the shortest string length.
Alternative approaches: divide-and-conquer, binary search, trie.
Testing strategy: include examples with varying lengths and no common prefix.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.