Pinterest·Software Engineer·Onsite - Coding / Algorithms·Senior

SeniorPrefer not to say

Jul 2026

Summary

Pinterest coding round, one question the whole time, and it was a string multiplication problem that spiraled into a full algorithms lecture. Not what I expected from a product company but they clearly wanted to see if you knew your theory.

Questions Asked (1)

Implement a function that multiplies two non-negative integers represented as decimal strings and returns the product as a string, without converting the inputs to any built-in big-integer type or using an arbitrary-precision library. Handle edge cases like zero inputs, leading zeros, and very large inputs efficiently. Discuss the trade-offs between grade-school O(n*m) multiplication, Karatsuba, and FFT/NTT-based approaches, and state the time and space complexity of your solution.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I went straight to grade-school long multiplication, which felt right, but I fumbled the carry propagation for a bit and had to restart that part.

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AI HintsAI Generated

Suggested Approach

Start by clarifying constraints and edge cases, then implement a grade-school multiplication using digit arrays or strings, and finally discuss advanced algorithms like Karatsuba and FFT/NTT with their trade-offs. Emphasize that for typical interview constraints, the O(n*m) approach is sufficient, but show awareness of when and why to use more complex methods.

Pro tip: Mention that you would first check if either input is '0' and return '0' immediately, and strip leading zeros to avoid unnecessary computation. Also, note that for very large inputs, you might consider chunking digits into larger bases (e.g., base 10^4) to reduce the number of operations and improve constant factors.

1. Clarify requirements and edge cases

Ask about input size limits, whether inputs can have leading zeros, and if the output should also be a string without leading zeros. Confirm that built-in big integer types are not allowed.

2. Design the algorithm

Choose an approach: grade-school O(n*m) for simplicity, or Karatsuba/FFT for very large inputs. Explain the trade-offs: grade-school is easy to implement and efficient for moderate sizes; Karatsuba reduces complexity to O(n^1.585) but has overhead; FFT/NTT achieves O(n log n) but is complex and has precision/implementation challenges.

3. Implement the chosen solution

For grade-school: convert strings to digit arrays, multiply digit by digit, accumulate results with carry, and convert back to string. Handle zero inputs and strip leading zeros from the result.

4. Analyze complexity and optimize

State time and space complexity: O(n*m) time and O(n+m) space for grade-school. Discuss potential optimizations like chunking digits or using Karatsuba/FFT for very large inputs, and mention their complexities.

5. Test and validate

Walk through test cases: zero inputs, single-digit, multi-digit, leading zeros, and large inputs. Verify correctness and performance.

Key Points to Mention

Edge cases: zero inputs, leading zeros, and empty strings.
Grade-school multiplication: O(n*m) time, O(n+m) space, simple and reliable.
Karatsuba algorithm: O(n^1.585) time, recursive divide-and-conquer, but overhead for small inputs.
FFT/NTT: O(n log n) time, complex implementation, precision issues with FFT, NTT requires modular arithmetic.
Trade-offs: choose based on input size and constraints; grade-school often sufficient for interviews.
Optimization: chunk digits into larger base (e.g., 10^4) to reduce operations and improve constant factors.

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