Pinterest·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Pinterest SWE interview with a string manipulation problem that had a sneaky follow-up. The core question was straightforward enough but the second part tripped me up a bit.

Questions Asked (1)

Given a numeric string, implement rounding to the nearest integer. For example, '3.5' should return '4' and '100.01' should return '100'. Follow-up: extend this to round to the last significant digit, so 124900 becomes 125000 and 12.49 becomes 12.5.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The base case wasn't bad.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the input format and edge cases (e.g., negative numbers, leading zeros, very long strings). For the base problem, parse the string to find the decimal point, compare the fractional part to 0.5, and adjust the integer part accordingly. For the follow-up, identify the last significant digit (first non-zero digit from the left) and round at that position, handling carries and trailing zeros.

Pro tip: Discuss how to handle very large numbers that exceed standard integer limits—suggest using string manipulation or arbitrary-precision arithmetic to avoid overflow. Also, mention that rounding half away from zero is a common convention, but clarify with the interviewer if ties should round up or to even.

1. Clarify requirements and edge cases

Ask about input format (e.g., can it have leading zeros, negative signs, or exponents?), expected output format, and tie-breaking rules. Confirm whether the string represents a decimal number and if rounding should be half-up, half-even, etc.

2. Design algorithm for basic rounding

Split the string at the decimal point. Compare the fractional part to '5' (or '0.5' if considering the whole fraction). If greater than or equal to 0.5, increment the integer part; otherwise, keep it. Handle carry-over (e.g., '9.9' -> '10').

3. Extend to rounding to last significant digit

Identify the last significant digit: the first non-zero digit from the left. Determine the rounding position (one place to the right of that digit). Round at that position, propagating carries and adjusting trailing zeros as needed.

4. Implement and test with examples

Write code (or pseudocode) for both parts, then walk through provided examples and additional edge cases (e.g., '0.5', '-3.5', '999.99', '0.000123', '124900'). Verify correctness and discuss time/space complexity.

5. Discuss trade-offs and optimizations

Compare string-based vs. numeric approaches, noting precision issues with floating-point. Mention potential optimizations like early termination or using regex for parsing, and consider scalability for very long strings.

Key Points to Mention

Handling edge cases: negative numbers, leading zeros, numbers without decimal points, and very large numbers that exceed integer limits.
String manipulation techniques: splitting, comparing substrings, and carrying over when rounding up (e.g., '9.9' -> '10').
Definition of 'last significant digit' and how to locate it (first non-zero digit from the left).
Tie-breaking rules: rounding half up vs. half to even, and the importance of clarifying with the interviewer.
Time and space complexity: O(n) time and O(n) space for string manipulation, with potential for O(1) extra space if modifying in place.
Trade-offs between string-based and numeric approaches: avoiding floating-point precision errors and overflow issues.

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