Google·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

May 2026

Summary

Coding round at Google for an ML Engineer role, one algorithmic problem that looked approachable until the O(N) constraint came up. The problem was more about seeing the pattern than knowing a specific algorithm.

Questions Asked (1)

Given an integer array, a 'good subarray' is one where every pair of adjacent elements differs by exactly 1 (either +1 or -1). Compute the sum of the element sums across all such good subarrays. The solution must run in O(N) time.

Algorithms & Data Structures

Author's notes

My first instinct was to enumerate subarrays and it took me an embarrassingly long time to accept that wasn't going to hit O(N).

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AI HintsAI Generated

Suggested Approach

Use a linear scan to identify maximal good subarrays by checking adjacent differences. For each maximal good subarray of length L, compute the sum of all its subarray sums using a contribution technique, then sum these across all maximal good subarrays.

Pro tip: Emphasize that the O(N) solution relies on decomposing the array into maximal good subarrays and using a formula for the sum of all subarray sums within each, avoiding nested loops. This demonstrates both algorithmic insight and optimization skill.

1. Identify maximal good subarrays

Scan the array once and split it into maximal contiguous segments where each adjacent pair differs by exactly 1. Each segment is a maximal good subarray.

2. Derive formula for sum of subarray sums

For a segment of length L with elements a[0..L-1], the sum of all subarray sums is Σ_{i=0}^{L-1} a[i] * (i+1) * (L-i). This can be computed in O(L) time.

3. Compute and accumulate

For each maximal good subarray, compute its total contribution using the formula and add it to the overall answer.

4. Handle edge cases

Consider arrays of length 0 or 1, and ensure the algorithm works when no good subarray of length >1 exists.

Key Points to Mention

Time complexity: O(N) because each element is visited a constant number of times.
Space complexity: O(1) extra space, as we only need a few variables.
The decomposition into maximal good subarrays is valid because any good subarray is contained within exactly one maximal good subarray.
The formula for sum of subarray sums uses the contribution of each element to all subarrays containing it.
Edge cases: empty array, single element, and arrays with no adjacent difference of 1.
Potential pitfalls: integer overflow if sums are large; use appropriate data types.

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