Leetcode #2199: Finding the Topic of Each Post
In this guide, we solve Leetcode #2199 Finding the Topic of Each Post 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: Keywords +-------------+---------+ | Column Name | Type | +-------------+---------+ | topic_id | int | | word | varchar | +-------------+---------+ (topic_id, word) is the primary key (combination of columns with unique values) for this table. Each row of this table contains the id of a topic and a word that is used to express this topic.
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 |
+-------------+---------+
| topic_id | int |
| word | varchar |
+-------------+---------+
(topic_id, word) is the primary key (combination of columns with unique values) for this table.
Each row of this table contains the id of a topic and a word that is used to express this topic.
There may be more than one word to express the same topic and one word may be used to express multiple topics.
Python Solution
import duckdb
import pandas as pd
def solution(keywords: pd.DataFrame, posts: pd.DataFrame) -> pd.DataFrame:
con = duckdb.connect()
con.register("Keywords", keywords)
con.register("Posts", posts)
return con.execute("""SELECT
post_id,
IFNULL(GROUP_CONCAT(DISTINCT topic_id), 'Ambiguous!') AS topic
FROM
Posts
LEFT JOIN Keywords ON INSTR(CONCAT(' ', content, ' '), CONCAT(' ', word, ' ')) > 0
GROUP BY post_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.