Mixpanel·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

Mixpanel software engineer interview with a pretty gnarly string arithmetic problem. No fluff, just one meaty coding question that took up the whole session.

Questions Asked (1)

Given two large decimal numbers as strings (no scientific notation, possible leading/trailing zeros), implement their exact sum as a string without using floating-point arithmetic or any big-number library. You need to handle alignment of fractional parts, carry propagation across the decimal point, and output normalization. Also discuss time/space complexity and cover edge cases like mismatched fractional lengths, integers mixed with decimals, all-zero inputs, and very large inputs.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

This one took me longer to get into than I expected.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then outline a digit-by-digit addition algorithm that aligns the decimal point and propagates carries. Emphasize normalization of the result and analyze time/space complexity, noting O(n) time and space where n is the total number of digits.

Pro tip: Mention that you can avoid string reversal by using two pointers from the end of each string, which simplifies carry handling and reduces overhead. Also, explicitly state that you will test with cases like '0.0' + '0.00' to ensure normalization returns '0'.

1. Clarify requirements and edge cases

Ask about input constraints (e.g., max length, sign handling) and confirm expected output format. List edge cases like mismatched fractional lengths, integers with decimals, all zeros, and very large inputs.

2. Design the algorithm

Split each number into integer and fractional parts. Pad the shorter fractional part with zeros to align. Perform addition from right to left, handling carry across the decimal point. Build the result string.

3. Implement normalization

After addition, remove leading zeros from the integer part (but keep at least one zero) and trailing zeros from the fractional part. If the fractional part becomes empty, omit the decimal point.

4. Analyze complexity

State that time complexity is O(n) where n is the maximum length of the input strings, and space complexity is O(n) for the result. Mention that no extra space beyond the output is needed if done in-place.

5. Test and validate

Walk through examples covering edge cases: '1.23' + '4.567', '123' + '0.456', '0.0' + '0.00', and very long strings. Verify carry propagation and normalization.

Key Points to Mention

Alignment of fractional parts by padding with zeros
Carry propagation across the decimal point
Normalization: trimming leading zeros in integer part and trailing zeros in fractional part
Handling of integers without decimal points
Time and space complexity analysis (O(n) time, O(n) space)
Edge cases: all-zero inputs, very large inputs, mismatched fractional lengths

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