Finding the Users Active Minutes

Finding the Users Active Minutes

1. What is this problem about?

The Finding the Users Active Minutes interview question is a data analysis challenge. You are given a list of user logs, where each log contains a user_id and a time (minute) of an action. A "User Active Minute" (UAM) is a unique minute during which a user performed at least one action. Your task is to calculate the UAM for each user and then return an array where the value at index $i$ is the number of users who had a UAM of exactly $i+1$.

2. Why is this asked in interviews?

Companies like Twitter and Amazon ask the Finding the Users Active Minutes coding problem to evaluate a candidate's ability to handle data deduplication and frequency counting. It tests your proficiency with Hash Table interview patterns, specifically nested structures like a map of sets. It evaluations if you can efficiently process log data to extract higher-level statistics.

3. Algorithmic pattern used

This problem follows the Deduplication and Frequency Counting pattern.

1. Deduplicate Logs: Use a Hash Map where the key is user_id and the value is a Hash Set of unique minutes. This automatically handles multiple actions in the same minute.
2. Calculate UAMs: Iterate through the map. For each user, their UAM is the size of their corresponding set.
3. Frequency Count: Create a result array of size k. Iterate through the UAM values and increment the count at result[uam - 1].

4. Example explanation

Logs: [[1, 5], [2, 2], [1, 5], [1, 2], [2, 2]], $k=5$.

1. User 1: Minutes {5, 5, 2}. Unique: {5, 2}. UAM = 2.
2. User 2: Minutes {2, 2}. Unique: {2}. UAM = 1.
3. UAM Counts: One user with UAM 1, one user with UAM 2. Result: [1, 1, 0, 0, 0].

5. Common mistakes candidates make

• Double Counting: Failing to use a Set to handle multiple logs for the same user in the same minute.
• Off-by-one: Mistakes in the result array indexing (index 0 corresponds to UAM 1).
• Efficiency: Using a list and searching it ($O(N^2)$) instead of a Hash Map/Set ($O(N)$).

6. Interview preparation tip

Whenever you hear "unique occurrence" or "deduplication," immediately think of a Set. Mastering the "Map of Sets" pattern is a vital Hash Table interview pattern for data analysis questions.