Leetcode #2688: Find Active Users
In this guide, we solve Leetcode #2688 Find Active Users in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Table: Users +-------------+----------+ | Column Name | Type | +-------------+----------+ | user_id | int | | item | varchar | | created_at | datetime | | amount | int | +-------------+----------+ This table may contain duplicate records. Each row includes the user ID, the purchased item, the date of purchase, and the purchase amount.
Quick Facts
- Difficulty: Medium
- Premium: Yes
- Tags: Database
Intuition
The task is relational in nature, which maps cleanly to DataFrame operations in Python.
By treating tables as DataFrames, joins and group-bys become concise and readable.
Approach
Load the inputs as DataFrames and apply the appropriate merge, filter, or group-by.
Select or rename the columns to match the required output.
Steps:
- Load inputs as DataFrames.
- Apply merge/groupby/filter operations.
- Select the output columns.
Example
+-------------+----------+
| Column Name | Type |
+-------------+----------+
| user_id | int |
| item | varchar |
| created_at | datetime |
| amount | int |
+-------------+----------+
This table may contain duplicate records.
Each row includes the user ID, the purchased item, the date of purchase, and the purchase amount.
Python Solution
import duckdb
import pandas as pd
# Pass input tables as keyword arguments matching the SQL table names.
def solution(**tables) -> pd.DataFrame:
con = duckdb.connect()
for name, df in tables.items():
con.register(name, df)
return con.execute("""SELECT DISTINCT
user_id
FROM Users
WHERE
user_id IN (
SELECT
user_id
FROM
(
SELECT
user_id,
created_at,
LAG(created_at, 1) OVER (
PARTITION BY user_id
ORDER BY created_at
) AS prev_created_at
FROM Users
) AS t
WHERE DATEDIFF(created_at, prev_created_at) <= 7
);""").df()
Complexity
The time complexity is O(n log n) (typical). The space complexity is O(n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.