Question 1

Implement a function that parses a flat list of comments (each with an id and parent_id) into a nested tree structure and renders it as indented text.

Accepted Answer

Start by building a map from id to node. Then do a single pass to attach children to parents, collecting roots (parent_id == null). Finally, DFS the tree to produce indented output. Walk through edge cases: cycles, missing parents, empty input. Use a hash map for O(n) time. What interviewers look for: Clean use of hash maps, awareness of edge cases, and ability to translate a recursive tree structure into code. This mirrors real feed/comment UI work, so they also watch whether you think about how this data would be displayed on screen.

Question 2

Given a list of app notifications with timestamps, implement a function that groups them into 'Today', 'Yesterday', and 'Older' buckets and returns them in that order.

Accepted Answer

Use the platform date API (Calendar on iOS, Calendar on Android) to compute the start-of-day boundaries for today and yesterday relative to the current time. Iterate notifications once, bucketing by timestamp comparison. Return a list of sections preserving insertion order within each bucket. Discuss timezone handling explicitly. What interviewers look for: Correct date arithmetic without off-by-one errors, awareness of timezone pitfalls, and the habit of not hardcoding 86400 seconds as a day. This is a realistic mobile data-transformation problem.

Question 3

Implement a simple LRU cache with get and put operations, then explain how you would use it for image caching in a mobile app.

Accepted Answer

Use a doubly linked list plus a hash map. get promotes the node to the head; put evicts the tail when over capacity. For the mobile extension: discuss memory vs. disk tiers, why you'd bound cache size by memory (MB not item count), and what happens on a low-memory warning. What interviewers look for: Correct LRU implementation is table stakes. The mobile extension reveals whether the candidate connects CS fundamentals to real platform problems — a key signal for mobile-specific roles.

Question 4

Walk me through the Android Activity lifecycle or the iOS UIViewController lifecycle. What happens when the user receives a phone call while using your app?

Accepted Answer

For Android: onCreate → onStart → onResume (foreground), then onPause → onStop (call arrives), onRestart → onResume (call ends). Explain that onPause is where you should pause media/sensors, and onStop is where you save state. For iOS: viewDidLoad → viewWillAppear → viewDidAppear, then viewWillDisappear on interruption. Mention that on iOS, applicationWillResignActive fires. What interviewers look for: Whether you know the exact sequence, not just the names. Interviewers listen for whether you can reason about what code goes where (e.g., releasing camera in onPause, not onStop) rather than reciting the diagram.

Question 5

What is the difference between a strong reference and a weak reference? Give an example of a retain cycle in a mobile app and how you would fix it.

Accepted Answer

Strong = keeps object alive; weak = does not prevent deallocation. Classic retain cycle: a ViewController holds a closure (e.g., a network callback) that captures self strongly. Fix: capture list [weak self] in Swift, or WeakReference / lambda capture on Android. Draw the reference graph. Also mention unowned vs. weak in Swift. What interviewers look for: Candidates who only recite the definition without drawing the cycle have surface-level knowledge. Interviewers want to see you construct the problematic reference graph and diagnose it, since retain cycles are a real new-grad pitfall in production apps.

Question 6

How does threading work in mobile apps? What goes on the main thread and why? What happens if you perform a network request on the main thread?

Accepted Answer

The main/UI thread processes all UI events and renders frames at 60fps — blocking it causes dropped frames or ANR (Android) / spinning beachball (iOS). Network I/O goes on background threads. On iOS: use DispatchQueue.global for background work, DispatchQueue.main.async to update UI. On Android: use Coroutines (Dispatchers.IO / Dispatchers.Main) or RxJava. Explain what ANR is and how to diagnose it with StrictMode or Instruments. What interviewers look for: Understanding of why the main thread constraint exists (the render loop), not just that 'you should use background threads.' Candidates who know the frame budget (16ms at 60fps) stand out.

Question 7

Design the local data layer for a simple Twitter-like feed app. The feed should load instantly on relaunch even without network.

Accepted Answer

Define the entities (Tweet, User). Use a local database (Room on Android, Core Data or SQLite on iOS) as the source of truth. On launch, serve cached data immediately while firing a network refresh in the background. Discuss cache invalidation: TTL, max row count, or server-driven expiry. Consider an offline-first architecture where writes are queued locally. Keep the design scoped — no need to design the server. What interviewers look for: At the new grad level, interviewers want to see you structure the problem (data model → storage → fetch strategy → staleness) rather than arrive at a perfect solution. Comfort with the concept of a local database as source of truth is a green flag.

Question 8

How would you build an image loading library from scratch? What components does it need?

Accepted Answer

Break it into: URL-to-image pipeline (download, decode, transform), a two-tier cache (in-memory LRU + disk cache keyed by URL hash), a request deduplication layer (same URL in-flight once), and a cancellation mechanism tied to view lifecycle. Discuss why you decode on a background thread and resize to display size before caching. Mention how existing libraries (Glide, Picasso, Kingfisher, SDWebImage) handle this. What interviewers look for: This is a favorite new-grad mobile design question because it spans networking, caching, threading, and UI — all areas relevant to the role. Interviewers check whether you identify all four components unprompted.

Question 9

Tell me about a mobile project you built — a class project, internship feature, or personal app. What was the hardest technical problem you hit?

Accepted Answer

Use STAR loosely but lead with the technical substance. Name the specific bug or constraint (e.g., 'layout was recalculating on every scroll frame, causing 40ms drops'). Explain what you tried first, why it didn't work, and what you ultimately did. Close with what you would do differently. Avoid vague answers like 'it was a teamwork challenge.' What interviewers look for: Signal that you go deep on problems rather than stopping at the first working solution. Interviewers want evidence of technical curiosity and debugging process, not just a good outcome.

Question 10

Describe a time you received feedback on your code — in a review or from a mentor — that changed how you write code.

Accepted Answer

Be specific: name the pattern or mistake (e.g., 'my reviewer pointed out I was force-unwrapping optionals throughout the codebase instead of handling nil gracefully'). Explain what you understood after the feedback. Show that you applied it going forward with a concrete example. What interviewers look for: Coachability and self-awareness. At the new grad level this is one of the highest-signal behavioral questions. Candidates who can't recall any meaningful feedback, or who are defensive, are a concern. Candidates who show genuine reflection are a green flag.

Question 11

What is the time and space complexity of a breadth-first search on a graph with V vertices and E edges? When would you use BFS vs. DFS in a mobile context?

Accepted Answer

BFS: O(V + E) time, O(V) space (queue). DFS: O(V + E) time, O(V) space (stack/call stack). Mobile context examples: BFS for shortest path in a navigation graph or finding the closest item in a tree of UI components; DFS for traversing a view hierarchy to find a specific view type, or walking a dependency tree. What interviewers look for: Correct complexity analysis delivered with confidence, plus the ability to connect abstract CS to concrete mobile scenarios. Many new grads know BFS/DFS but stumble when asked to apply it outside LeetCode problems.

Question 12

What is the difference between a process and a thread? How does this apply to how mobile OSes handle app background execution?

Accepted Answer

Process = isolated memory space, OS-managed. Thread = execution unit within a process, shared memory. On iOS, apps are suspended in the background within seconds unless they request background modes (audio, location, background fetch). On Android, processes can be killed by the OS under memory pressure, and you should use WorkManager for deferrable background work. Relate this back to why you can't rely on background threads for critical long-running work. What interviewers look for: Whether you understand the OS-level constraint, not just the textbook definition. New grads who have only done server-side work often assume background threads run indefinitely — this question surfaces that gap.

New Grad Mobile Engineer Interview Questions

What to expect

12 questions, with how to answer them

1. Implement a function that parses a flat list of comments (each with an id and parent_id) into a nested tree structure and renders it as indented text.

2. Given a list of app notifications with timestamps, implement a function that groups them into 'Today', 'Yesterday', and 'Older' buckets and returns them in that order.

3. Implement a simple LRU cache with get and put operations, then explain how you would use it for image caching in a mobile app.

4. Walk me through the Android Activity lifecycle or the iOS UIViewController lifecycle. What happens when the user receives a phone call while using your app?

5. What is the difference between a strong reference and a weak reference? Give an example of a retain cycle in a mobile app and how you would fix it.

6. How does threading work in mobile apps? What goes on the main thread and why? What happens if you perform a network request on the main thread?

7. Design the local data layer for a simple Twitter-like feed app. The feed should load instantly on relaunch even without network.

8. How would you build an image loading library from scratch? What components does it need?

9. Tell me about a mobile project you built — a class project, internship feature, or personal app. What was the hardest technical problem you hit?

10. Describe a time you received feedback on your code — in a review or from a mentor — that changed how you write code.

11. What is the time and space complexity of a breadth-first search on a graph with V vertices and E edges? When would you use BFS vs. DFS in a mobile context?

12. What is the difference between a process and a thread? How does this apply to how mobile OSes handle app background execution?

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