Leetcode #1809: Ad-Free Sessions
In this guide, we solve Leetcode #1809 Ad-Free Sessions 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: Playback +-------------+------+ | Column Name | Type | +-------------+------+ | session_id | int | | customer_id | int | | start_time | int | | end_time | int | +-------------+------+ session_id is the column with unique values for this table. customer_id is the ID of the customer watching this session.
Quick Facts
- Difficulty: Easy
- 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 |
+-------------+------+
| session_id | int |
| customer_id | int |
| start_time | int |
| end_time | int |
+-------------+------+
session_id is the column with unique values for this table.
customer_id is the ID of the customer watching this session.
The session runs during the inclusive interval between start_time and end_time.
It is guaranteed that start_time <= end_time and that two sessions for the same customer do not intersect.
Python Solution
import duckdb
import pandas as pd
def solution(playback: pd.DataFrame, ads: pd.DataFrame) -> pd.DataFrame:
con = duckdb.connect()
con.register("Playback", playback)
con.register("Ads", ads)
return con.execute("""SELECT session_id
FROM Playback
WHERE
session_id NOT IN (
SELECT session_id
FROM
Playback AS p
JOIN Ads AS a
ON p.customer_id = a.customer_id AND a.timestamp BETWEEN p.start_time AND p.end_time
);""").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.