Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026

Summary

Meta SWE coding round, one question the whole time. Sliding window on non-negative integers, seemed straightforward but the constraints are wide enough that you have to actually think about the approach.

Questions Asked (1)

Given an array of non-negative integers and a target value, determine whether any contiguous subarray sums to exactly that target. Return true or false.

Algorithms & Data Structures

Author's notes

My first instinct was prefix sums with a hash set, which works when you have negative numbers in the mix.

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AI HintsAI Generated

Suggested Approach

Start by clarifying constraints (e.g., array size, whether negative numbers are allowed) and then propose an efficient solution using a hash map to track prefix sums, achieving O(n) time. Explain the algorithm step-by-step, handle edge cases, and analyze time and space complexity.

Pro tip: Mention that if the array contains only non-negative numbers, a sliding window approach works, but the prefix sum method is more general and handles negative numbers too. This shows you understand trade-offs and can adapt to variations.

1. Clarify the problem

Ask about constraints: array size, range of values, whether negative numbers are allowed, and if the subarray must be non-empty. This ensures you understand the problem fully before coding.

2. Discuss brute force and optimal approach

Mention the O(n^2) brute force method, then propose the O(n) prefix sum with hash map approach. Explain why it's more efficient and how it works.

3. Explain the algorithm

Describe maintaining a running sum and a hash map of prefix sums to their earliest index. For each element, check if (current_sum - target) exists in the map; if so, return true. Otherwise, store the current sum if not already present.

4. Walk through an example

Choose a small example (e.g., [1,2,3], target=5) and trace the algorithm step-by-step to demonstrate correctness and clarity.

5. Analyze complexity and edge cases

State time complexity O(n) and space complexity O(n). Discuss edge cases: empty array, target=0, all zeros, and large arrays.

Key Points to Mention

Prefix sum technique and its use in subarray sum problems
Hash map for O(1) lookups to achieve linear time
Handling of zero and negative numbers (if allowed)
Time and space complexity analysis
Edge cases such as empty array, target=0, and non-contiguous elements
Comparison with sliding window approach for non-negative arrays

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