Pinterest·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jul 2026

Summary

Pinterest SWE interview with a string manipulation problem that looks dead simple but has a few layers once they start asking follow-ups. The core coding part was fine, the discussion afterward is where it got interesting.

Questions Asked (1)

Given two strings, determine whether the target string can be constructed using characters from the source string, where each character in the source can only be used as many times as it appears.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I went straight to the frequency map approach, decrement counts as you consume characters, return false if anything dips below zero.

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AI HintsAI Generated

Suggested Approach

Clarify that this is a character frequency matching problem, then propose using a hash map or fixed-size array to count characters in the source and decrement for the target. Discuss trade-offs between sorting, hash maps, and arrays, and analyze time and space complexity.

Pro tip: Mention that for ASCII or Unicode strings, a fixed-size array can be more efficient than a hash map, but clarify assumptions about the character set. Also, consider edge cases like empty strings and early termination when counts go negative.

1. Clarify the problem

Ask if the strings are case-sensitive, what character set is used (ASCII, Unicode), and whether the target can be empty. Confirm that each source character can be used at most as many times as it appears.

2. Choose a data structure

Decide between a hash map (for general character sets) or a fixed-size array (for ASCII). Explain the trade-offs in terms of time and space complexity.

3. Count source characters

Iterate through the source string and increment the count for each character in the chosen data structure.

4. Check target characters

Iterate through the target string, decrement the count for each character, and if any count becomes negative, return false immediately.

5. Return result and analyze

If all characters are processed successfully, return true. Analyze the time complexity O(n + m) and space complexity O(k) where k is the number of unique characters.

Key Points to Mention

Time complexity: O(n + m) where n and m are lengths of source and target.
Space complexity: O(k) where k is the number of unique characters (or O(1) for fixed character set).
Trade-offs between hash map and array: hash map is more flexible for Unicode but has overhead; array is faster for ASCII.
Edge cases: empty strings, target longer than source, characters not in source.
Early termination: return false as soon as a character count goes negative.
Alternative approaches: sorting both strings and comparing (O(n log n) time, O(1) space if in-place) but less efficient.

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