Leetcode #2228: Users With Two Purchases Within Seven Days
In this guide, we solve Leetcode #2228 Users With Two Purchases Within Seven Days 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: Purchases +---------------+------+ | Column Name | Type | +---------------+------+ | purchase_id | int | | user_id | int | | purchase_date | date | +---------------+------+ purchase_id contains unique values. This table contains logs of the dates that users purchased from a certain retailer.
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 |
+---------------+------+
| purchase_id | int |
| user_id | int |
| purchase_date | date |
+---------------+------+
purchase_id contains unique values.
This table contains logs of the dates that users purchased from a certain retailer.
Python Solution
import duckdb
import pandas as pd
def solution(purchases: pd.DataFrame) -> pd.DataFrame:
con = duckdb.connect()
con.register("Purchases", purchases)
return con.execute("""WITH
t AS (
SELECT
user_id,
DATEDIFF(
purchase_date,
LAG(purchase_date, 1) OVER (
PARTITION BY user_id
ORDER BY purchase_date
)
) AS d
FROM Purchases
)
SELECT DISTINCT user_id
FROM t
WHERE d <= 7
ORDER BY user_id;""").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.