Pinterest·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Pinterest phone screen for a software engineering role, one coding question the whole time. Pretty focused on string parsing and knowing when to ask clarifying questions.

Questions Asked (1)

Given a string of even length containing only digit characters, decode it by reading consecutive pairs where the first character is a repeat count and the second is the digit to repeat. Return the fully expanded string.

Algorithms & Data StructuresAdaptability & Ambiguity

Author's notes

Basically the reverse of the classic count-and-say problem.

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AI HintsAI Generated

Suggested Approach

Clarify the decoding rules and edge cases, then propose an efficient algorithm that processes the string in pairs. Discuss time and space complexity, and consider how to handle invalid inputs or large outputs.

Pro tip: Demonstrate adaptability by discussing how you would handle ambiguous cases, such as a repeat count of 0 or invalid characters, and suggest ways to extend the solution for streaming input.

1. Understand the problem

Confirm the input format: a string of even length with digit characters, where each pair consists of a repeat count (first digit) and a digit to repeat (second digit). Clarify that the repeat count is a single digit (0-9) and the output should be the expanded string.

2. Plan the algorithm

Iterate through the string in steps of 2. For each pair, parse the repeat count and the digit, then append the digit repeated that many times to a result builder. Use a StringBuilder for efficiency.

3. Handle edge cases

Consider cases like repeat count 0 (which produces no output), empty string (though length is even, it could be 0), and invalid characters (non-digits). Discuss whether to assume valid input or add validation.

4. Analyze complexity

Time complexity is O(n) where n is the length of the input string, as we process each character once. Space complexity is O(m) where m is the length of the output string, which could be up to 9 times the number of pairs.

5. Test with examples

Walk through a simple example like '2a3b' (if digits were allowed) or '2132' to verify the logic. For '2132', the output should be '1122' (1 repeated 2 times, then 3 repeated 2 times).

Key Points to Mention

Clarify that the repeat count is a single digit (0-9) and the digit to repeat is also a single digit.
Use a StringBuilder to efficiently build the output string.
Discuss time and space complexity: O(n) time, O(m) space.
Consider edge cases: repeat count 0, empty string, invalid characters.
Mention potential optimizations for large outputs, such as streaming or chunked processing.
Demonstrate adaptability by discussing how to handle ambiguous or invalid input.

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