Microsoft·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Microsoft coding interview, one question on string multiplication. Pretty standard algorithmic problem but the no-conversion constraint is the whole point of it.

Questions Asked (1)

Given two non-negative integers represented as strings, return their product as a string. You cannot convert the inputs to a numeric type directly.

Algorithms & Data Structures

Author's notes

The constraint is what makes it annoying.

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AI HintsAI Generated

Suggested Approach

Simulate grade-school multiplication by processing digits from right to left, accumulating results in an array where each position represents a digit of the product. Handle carries and leading zeros carefully, then convert the array to a string. This avoids direct numeric conversion and works for arbitrarily large numbers.

Pro tip: Clarify edge cases upfront (e.g., inputs '0', empty strings) and discuss time/space complexity (O(m*n) time, O(m+n) space). Mention that this approach is essentially what BigInteger libraries do internally, showing depth of understanding.

1. Clarify requirements and edge cases

Confirm input constraints (non-negative, no leading zeros except '0'), output format, and handling of zero. Ask about maximum input length to gauge if optimization is needed.

2. Choose data structure and initialize

Use an integer array of size m+n (where m and n are lengths of inputs) to store intermediate results. Initialize all elements to 0.

3. Perform digit-by-digit multiplication

Iterate from the least significant digit of both strings. For each pair, multiply digits, add to the current position, and propagate carry to the next position.

4. Convert result to string and handle leading zeros

Build the result string from the array, skipping leading zeros. If the result is empty, return '0'.

5. Analyze complexity and test

State time complexity O(m*n) and space O(m+n). Walk through a small example (e.g., '12' * '34') to verify correctness.

Key Points to Mention

Grade-school multiplication algorithm
Handling carries and overflow
Using an array to store intermediate results
Time and space complexity analysis
Edge cases: zero, leading zeros, empty strings
Avoiding direct numeric conversion

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