Pinterest·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Pinterest coding screen for a software engineer role. Just the one problem but it was a bit of a weird one if you haven't seen it before.

Questions Asked (1)

Implement the count-and-say sequence: given a positive integer n, return the nth term as a string, where each term is built by run-length encoding the previous term.

Algorithms & Data Structures

Author's notes

I knew what run-length encoding was but hadn't connected it to this problem before.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the definition of the count-and-say sequence and the base case (n=1 returns '1'). Then, iteratively build each term from the previous one by scanning and counting consecutive identical digits, converting the count and digit into a string. Focus on writing clean, efficient code and analyzing time and space complexity.

Pro tip: Demonstrate strong communication by walking through a small example (e.g., n=4) before coding, and mention potential optimizations like using a StringBuilder for string concatenation to avoid quadratic time. Also, discuss edge cases like n=1 and large n, and consider if the sequence can be generated recursively versus iteratively.

1. Clarify the problem and edge cases

Confirm the sequence definition, base case (n=1 -> '1'), and constraints (e.g., n up to 30). Ask if n is guaranteed positive and if the output should be a string.

2. Choose an iterative approach

Decide to build the sequence iteratively from 1 to n, as recursion may cause stack overflow for large n. Initialize the first term as '1'.

3. Implement run-length encoding

For each term from 2 to n, scan the previous term, count consecutive identical digits, and append the count followed by the digit to a new string (using StringBuilder for efficiency).

4. Analyze complexity and test

Discuss time complexity O(2^n) in the worst case (since the length grows exponentially) and space O(2^n) for the output. Test with small n (1,2,3,4) and edge cases.

Key Points to Mention

Base case: n=1 returns '1'.
Iterative vs recursive: iterative avoids stack overflow and is more efficient.
Run-length encoding: count consecutive identical digits and append count then digit.
Use StringBuilder for efficient string concatenation.
Time and space complexity: exponential growth due to sequence length doubling roughly each term.
Edge cases: n=1, large n, and potential integer overflow if using recursion.

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