Pinterest·Machine Learning Engineer·Technical Phone Screen·Senior

Senior

Jul 2026

Summary

Pinterest MLE interview with a string manipulation problem that sounds deceptively simple until you realize floats will absolutely betray you. The constraint about not using float() or Decimal was the whole point.

Questions Asked (1)

Given two numeric strings, one representing a value and one representing a precision, return the value rounded to that precision as a string. For example, rounding '12567' to the nearest '100' gives '12600', or rounding '1234.678' to '0.1' gives '1234.7'. You cannot use float() or Decimal helpers; implement the rounding directly on the string representations.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was to just cast to float and call it a day, and the interviewer shut that down immediately.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and edge cases first, then outline a string-based algorithm that aligns the precision to the value's decimal representation. Implement the rounding by examining the digit after the rounding position, handling carries and trailing zeros, and finally validate with examples.

Pro tip: Mention that you would test edge cases like negative numbers, exact halfway values, and precision larger than the value's magnitude, and discuss how to handle them consistently (e.g., round half up).

1. Clarify requirements and edge cases

Ask about negative numbers, rounding mode (half-up, half-even), precision formats (e.g., '100', '0.1'), and whether the result should preserve trailing zeros. Confirm that the precision is always a power of 10.

2. Normalize representations

Convert both strings to a common format: remove decimal points by scaling to integers, or align decimal places. Determine the rounding position relative to the value's decimal point.

3. Implement rounding logic

Identify the digit at the rounding position and the next digit. If the next digit is >=5, increment the rounding digit and propagate carries leftward; otherwise, truncate. Handle decimal point placement and trailing zeros.

4. Handle special cases

Address cases where precision is larger than the value (result is 0 or the value itself), negative numbers (apply rounding to absolute value and reapply sign), and exact halfway values according to the chosen rounding mode.

5. Test and validate

Walk through provided examples and additional edge cases (e.g., '999' rounded to '10' -> '1000', '0.5' rounded to '1' -> '1'). Verify that the output is a string with correct formatting.

Key Points to Mention

Avoid floating-point arithmetic entirely; operate on strings or integers derived from strings.
Determine the rounding position by comparing the precision's exponent to the value's decimal point.
Implement carry propagation when rounding up (e.g., '9' becomes '0' and carry 1).
Handle negative numbers by rounding the absolute value and reapplying the sign.
Consider rounding modes (half-up, half-even) and clarify with the interviewer.
Ensure the output string preserves the correct number of decimal places as implied by the precision.

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