Roblox·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Roblox software engineer interview with a pretty gnarly algorithm question about music player modes. The problem felt deceptively clean on the surface but had enough edge cases to keep me on my toes for a while.

Questions Asked (1)

You have a playlist of distinct songs and two playback modes: Random (picks next song uniformly at random with replacement) and Shuffle (generates a random permutation and plays through it without repeats, then optionally reshuffles). Given a sequence of n songs recorded from an unknown starting point, design an algorithm to determine whether that sequence is consistent with Shuffle mode. Include time and space complexity analysis and cover edge cases like repeated songs and n=1.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I actually liked this problem once I got my head around it.

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AI HintsAI Generated

Suggested Approach

First, clarify the problem: a sequence is consistent with Shuffle if it contains no repeated songs within any contiguous block of length up to the total number of distinct songs, assuming the sequence is a contiguous segment of a single shuffle cycle. Then, design an algorithm that checks for duplicate songs within a sliding window of size equal to the number of distinct songs, using a hash set or map to track the last occurrence of each song. Analyze time and space complexity, and discuss edge cases such as repeated songs, n=1, and sequences longer than the number of distinct songs.

Pro tip: Demonstrate awareness that the sequence could start mid-shuffle, so the first cycle might be incomplete; also mention that if the sequence length exceeds the number of distinct songs, it must contain a full cycle and possibly part of the next, so the window size should be the total distinct count.

1. Clarify assumptions and constraints

Confirm that the playlist has distinct songs, the sequence is contiguous from an unknown starting point, and Shuffle mode plays a random permutation without repeats until reshuffling. Discuss whether reshuffling occurs immediately after a full cycle or only when the user requests it.

2. Define consistency condition

A sequence is consistent with Shuffle if no song repeats within any window of size equal to the total number of distinct songs in the playlist. This ensures that within a single shuffle cycle, all songs are played exactly once before any repeat.

3. Design algorithm

Use a hash map to store the last seen index of each song. Iterate through the sequence; if a song is seen again and the distance between occurrences is less than the total number of distinct songs, return false. Otherwise, update the last seen index. Return true if no violations.

4. Analyze complexity

Time complexity is O(n) because each song is processed once. Space complexity is O(k), where k is the number of distinct songs in the playlist, for the hash map.

5. Handle edge cases

For n=1, always consistent. If the sequence contains songs not in the playlist, it's inconsistent. If the sequence length exceeds the number of distinct songs, it must contain a full cycle and possibly part of the next; the window check still applies. Also consider sequences with no repeats but shorter than the full playlist—they are consistent.

Key Points to Mention

The total number of distinct songs in the playlist is the key parameter for the window size.
Use a hash map to track the last occurrence of each song for O(1) lookups.
The sequence can start at any point in a shuffle cycle, so the first cycle may be incomplete.
If the sequence length is greater than the number of distinct songs, it must contain at least one full cycle, so repeats are expected but must be separated by at least the total distinct count.
Edge cases: n=1, songs not in playlist, sequences with all unique songs but shorter than playlist.
Time complexity O(n), space complexity O(k) where k is the number of distinct songs.

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