Pinterest·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Pinterest coding screen, one algorithmic problem the whole time. Clean problem statement but the O(n) constraint is where it gets real.

Questions Asked (1)

Given an array of positive integers and a value K, count the number of subarrays where the product of the subarray sum and its length is less than or equal to K.

Algorithms & Data Structures

Author's notes

Took me longer than I'd like to admit to see why a sliding window works here.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem and constraints, then propose a sliding window approach that maintains the sum and product for a window, expanding and shrinking as needed. Explain how to efficiently count valid subarrays by leveraging monotonicity and avoiding redundant calculations.

Pro tip: Mention that since all numbers are positive, the product of sum and length is monotonically increasing as the window expands, which justifies the two-pointer technique. Also, discuss potential integer overflow and how to handle it (e.g., using long integers or early termination).

1. Clarify the problem

Confirm that subarrays are contiguous, that the product is sum * length, and that we need to count subarrays with product <= K. Ask about constraints on array size and values to determine the optimal approach.

2. Identify the approach

Recognize that with positive integers, the product sum*length is monotonic with respect to window expansion. Propose a sliding window (two-pointer) technique to achieve O(n) time complexity.

3. Design the algorithm

Initialize left=0, sum=0, count=0. Iterate right from 0 to n-1, add arr[right] to sum. While sum * (right-left+1) > K, subtract arr[left] from sum and increment left. Then add (right-left+1) to count, as all subarrays ending at right with start >= left are valid.

4. Analyze complexity and edge cases

Explain that each element is added and removed at most once, so time is O(n) and space is O(1). Discuss edge cases: K=0 (no valid subarrays), large sums causing overflow, and empty array.

5. Test with examples

Walk through a small example, such as arr=[1,2,3], K=10, to demonstrate the algorithm and verify correctness. Mention that the count can be large, so use appropriate data types.

Key Points to Mention

Sliding window technique and its applicability due to positive integers
Monotonicity of the product sum*length as window expands
Time complexity O(n) and space complexity O(1)
Handling integer overflow with long integers or early break
Edge cases: K=0, empty array, large values
Counting all valid subarrays by adding window length at each step

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