Pinterest·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

May 2026

Summary

Pinterest ML Engineer interview with a string multiplication problem that sounds easy until you remember you can't just cast to int and multiply. Straightforward round but the constraint is the whole point.

Questions Asked (1)

Given two non-negative integers represented as strings, return their product also as a string. You cannot use any BigInteger library or convert the inputs to integers directly.

Algorithms & Data Structures

Author's notes

The constraint is what makes this annoying.

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AI HintsAI Generated

Suggested Approach

Simulate grade-school multiplication digit by digit, storing intermediate results in an array where each position accumulates the product of corresponding digits. After processing all digit pairs, handle carries to produce the final string, ensuring no leading zeros except for the result '0'.

Pro tip: Mention that this approach is O(m*n) time and O(m+n) space, and note that it's the same algorithm used in arbitrary-precision arithmetic libraries, showing you understand the trade-offs and real-world applications.

1. Clarify constraints and edge cases

Confirm that inputs are non-negative, may be very large, and that the result should be a string. Discuss edge cases like '0' and '1'.

2. Choose data structure and algorithm

Use an integer array of size m+n to store intermediate results. Simulate multiplication from least significant digit to most significant.

3. Implement digit-by-digit multiplication

Iterate over each digit of the first number and second number, compute the product, and add it to the appropriate position in the array, handling carries.

4. Convert array to string

After all multiplications, process carries from right to left, then build the result string by skipping leading zeros.

5. Test and verify

Walk through examples like '123' * '456' and edge cases like '0' * '0' to ensure correctness.

Key Points to Mention

Time complexity O(m*n) and space complexity O(m+n)
Handling carries correctly during accumulation
Avoiding leading zeros in the final string
Edge cases: multiplication by zero, single-digit numbers
Comparison to BigInteger implementation and why it's needed
Potential optimizations like using base 10^9 for fewer operations

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