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Two Sigma·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jun 2026

Summary

Two Sigma phone screen, pretty much just one coding problem the whole time. The constraint of using a helper function instead of just parsing the strings made it trickier than it sounds on paper.

Questions Asked (1)

Q1

Given two non-negative numbers represented as strings, return their sum as a string. You must use a provided helper function that adds two single digits together and cannot convert the full strings to integers directly.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

My first instinct was to just parseInt both strings and add them, which is exactly what they said not to do.

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AI HintsAI Generated

Suggested Approach

Treat the strings as sequences of digits and simulate the addition process from right to left, using the provided helper to add individual digits along with any carry. Build the result incrementally, handling carries and different string lengths, then reverse or prepend to get the final sum string.

Pro tip: Clarify edge cases upfront, such as leading zeros, empty strings, or very long inputs, and discuss how your solution handles them without converting to integers. Mention that the helper function is the only allowed digit-level operation, showing you respect the constraint.

1. Clarify constraints and edge cases

Ask about input format (e.g., leading zeros, empty strings) and confirm that the helper function adds two single digits and returns the sum and carry. Discuss how to handle cases like '0' + '0' or strings of different lengths.

2. Plan the digit-by-digit addition

Use two pointers starting from the end of each string. At each step, extract the current digits (or 0 if one string is exhausted), call the helper to add them plus any carry, and record the resulting digit and new carry.

3. Build the result efficiently

Prepend each resulting digit to a result string or append to a list and reverse at the end. Avoid inefficient string concatenation in a loop; use a list or StringBuilder for O(n) time.

4. Handle remaining carry and finalize

After processing all digits, if there is a remaining carry, append it. Remove any leading zeros if necessary (unless the result is '0'). Return the final string.

5. Analyze complexity and trade-offs

State that time complexity is O(max(n, m)) and space is O(max(n, m)) for the result. Discuss alternative approaches (e.g., recursion) and why iterative is preferable for large inputs.

Key Points to Mention

  • Simulate addition from least significant digit to most significant, mimicking manual addition.
  • Use the provided helper function for each digit pair and carry, ensuring no direct integer conversion of the full strings.
  • Handle different string lengths by treating missing digits as 0.
  • Manage carry propagation correctly, including a final carry that may add an extra digit.
  • Optimize result construction using a list or StringBuilder to avoid O(n^2) string concatenation.
  • Discuss edge cases: empty strings, leading zeros, and very large numbers that exceed integer limits.

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