Leetcode #2994: Friday Purchases II
In this guide, we solve Leetcode #2994 **Friday Purchases II ** 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 | +---------------+------+ | user_id | int | | purchase_date | date | | amount_spend | int | +---------------+------+ (user_id, purchase_date, amount_spend) is the primary key (combination of columns with unique values) for this table. purchase_date will range from November 1, 2023, to November 30, 2023, inclusive of both dates.
Quick Facts
- Difficulty: Hard
- 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 |
| purchase_date | date |
| amount_spend | int |
+---------------+------+
(user_id, purchase_date, amount_spend) is the primary key (combination of columns with unique values) for this table.
purchase_date will range from November 1, 2023, to November 30, 2023, inclusive of both dates.
Each row contains user id, purchase date, and amount spend.
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 RECURSIVE
T AS (
SELECT '2023-11-01' AS purchase_date
UNION
SELECT purchase_date + INTERVAL 1 DAY
FROM T
WHERE purchase_date < '2023-11-30'
)
SELECT
CEIL(DAYOFMONTH(purchase_date) / 7) AS week_of_month,
purchase_date,
IFNULL(SUM(amount_spend), 0) AS total_amount
FROM
T
LEFT JOIN Purchases USING (purchase_date)
WHERE DAYOFWEEK(purchase_date) = 6
GROUP BY 2
ORDER BY 1;""").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.