Moloco Interview Questions — Page 1

Moloco·Software Engineer·Technical Phone Screen

Apr 2026

Interviewed for a software engineer role at Molocoads and got a range sum query problem that started simple and escalated fast. They walked through both the immutable and mutable versions, which meant knowing two completely different approaches under pressure.

Given an integer array, implement two operations: update a value at a given index, and return the sum of elements between two indices inclusive. Walk through both an immutable and mutable version, and compare the data structures you'd use for each.

“Started with the prefix sum approach since that's the obvious entry point, O(1) query but falls apart the moment updates come into the picture.”

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Moloco·Machine Learning Engineer·Onsite - System Design / Architecture

Apr 2026

Interviewed for an MLE role at Molocoads and the whole thing was basically a deep dive into recommendation system design. They wanted you to own every layer of the stack and defend your choices, not just recite a textbook answer.

Walk through how you would design a full recommendation system end to end, from feature engineering through online serving.
How do you handle candidate generation? Walk through collaborative filtering versus two-tower retrieval versus content-based approaches and when you'd pick each.
What does your ranking model architecture look like and how do you train it?
How do you approach re-ranking to account for diversity, freshness, and business constraints?
How do you evaluate a recommendation system offline versus online, and what metrics do you use for each?
How would you design an A/B test to validate a change to the recommendation model?
How do you deal with the cold start problem for new users and new items?

“This is one of those questions where you think you know it cold and then you open your mouth and realize you've been fuzzy on the transitions between stages.”

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Moloco·Data Scientist·Technical Phone Screen

Apr 2026

SQL-heavy screen for a data scientist role at Molocoads. Two questions, both involving multi-table joins and window functions. Nothing behavioral, just pure query writing under time pressure.

Given a table of daily ad spend by advertiser, find the top 3 spending advertisers for each day. Handle ties so that advertisers tied within rank 3 are all included. Return the date, advertiser ID, their total daily spend, and their rank.
For each campaign goal type, calculate the total spend, total KPI conversions, and cost per KPI. Only include spend and conversions that occurred on or after the campaign's launch date. Return NULL for cost per KPI if there were zero conversions.

“This is a DENSE_RANK vs ROW_NUMBER question in disguise.”

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Moloco·Software Engineer·Technical Phone Screen

May 2026

Interviewed for a software engineer role at Molocoads and got a string parsing problem that looked deceptively clean on the surface. Not a ton of context about the broader process, just this one problem to work through.

Given a valid parentheses string, compute its score where '()' equals 1, adjacent strings add their scores, and a string wrapped in outer parentheses doubles its inner score.

“I knew this was a stack problem pretty quickly but fumbled the doubling logic for nested cases.”

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Moloco·Software Engineer·Technical Phone Screen

May 2026

Interviewed at Molocoads for a software engineering role and got a string problem that looks deceptively simple until you think about the constraints. The brute force is obvious but won't pass, so knowing your trie fundamentals matters here.

Given two lists of strings A and B, pick one string from each list and return the maximum possible length of their longest common prefix.

“My first instinct was just brute force every pair and track the max prefix length.”

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Moloco·Software Engineer·Technical Phone Screen

May 2026

Did a coding round for a Software Engineer role at Molocoads. The problem looked deceptively simple at first glance but the follow-ups pushed into binary search territory which I wasn't fully prepared for.

You're given a sorted array where every value appears exactly k times in a row, except for one value that appears exactly once. Write a function to find and return that unique value.
What is the relationship between the array length n, the repetition count k, and the number of distinct repeated elements? Express n as a formula.
Can you redesign your solution to run in O(log n) time using binary search, taking advantage of the sorted grouping structure?

“My first instinct was a hash map and I almost said it out loud before catching myself.”

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Moloco·Software Engineer·Technical Phone Screen

Jun 2026

Molocoads coding round, one problem the whole time. Grid simulation with bubble elimination and gravity. Nothing flashy, just had to get the mechanics right without tripping over edge cases.

Given a 2D grid of colored bubbles, identify and eliminate groups of adjacent same-color cells that meet a size threshold, then apply gravity so remaining cells fall down and empty cells rise to the top. Return the resulting grid.

“The problem itself isn't hard conceptually but I kept second-guessing my BFS for the grouping step.”

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Moloco·Software Engineer·Technical Phone Screen

Jun 2026

Interviewed for a software engineer role at Molocoads and got a graph traversal problem about unlocking rooms. Pretty standard algorithmic question but it's the kind that trips you up if you overthink the setup.

You have n rooms labeled 0 to n-1, all locked except room 0. Each room contains keys to other rooms. Starting from room 0, can you visit every room? Given the list of keys per room, return true or false.

“Classic graph reachability problem.”

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Moloco·Software Engineer·Technical Phone Screen

Jun 2026

One round with a senior Korean interviewer who was genuinely pleasant despite a thick accent making some back-and-forth a bit tricky. The technical problem was deceptively simple on the surface but turned into a pretty long conversation about optimization.

You have a log file where each entry is a (user, ad-click) pair and a single user can appear many times. How do you compute the total number of clicks per user, and how do you optimize beyond a naive O(n) worst case?

“Started with the obvious grouping approach and the interviewer nodded along, then asked how to make the worst case better.”

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Moloco·Software Engineer·Technical Phone Screen

Jun 2026

Interviewed for a software engineer role at Molocoads and got hit with a pretty involved array problem that had a follow-up I was not expecting. The core question was manageable but the circular variant threw me off a bit.

Given an integer array, for each index i, find the maximum length of a contiguous subarray that contains i and whose maximum element is exactly nums[i].
Follow-up: how would you modify your solution if the array is circular, meaning the subarray can wrap around from the end back to the beginning?

“The key insight is that you're basically looking for how far left and right you can walk from i before hitting something strictly larger.”

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