Leetcode #1990: Count the Number of Experiments
In this guide, we solve Leetcode #1990 Count the Number of Experiments 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: Experiments +-----------------+------+ | Column Name | Type | +-----------------+------+ | experiment_id | int | | platform | enum | | experiment_name | enum | +-----------------+------+ experiment_id is the column with unique values for this table. platform is an enum (category) type of values ('Android', 'IOS', 'Web').
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 |
+-----------------+------+
| experiment_id | int |
| platform | enum |
| experiment_name | enum |
+-----------------+------+
experiment_id is the column with unique values for this table.
platform is an enum (category) type of values ('Android', 'IOS', 'Web').
experiment_name is an enum (category) type of values ('Reading', 'Sports', 'Programming').
This table contains information about the ID of an experiment done with a random person, the platform used to do the experiment, and the name of the experiment.
Python Solution
import duckdb
import pandas as pd
def solution(experiments: pd.DataFrame) -> pd.DataFrame:
con = duckdb.connect()
con.register("Experiments", experiments)
return con.execute("""WITH
P AS (
SELECT 'Android' AS platform
UNION
SELECT 'IOS'
UNION
SELECT 'Web'
),
Exp AS (
SELECT 'Reading' AS experiment_name
UNION
SELECT 'Sports'
UNION
SELECT 'Programming'
),
T AS (
SELECT *
FROM
P,
Exp
)
SELECT platform, experiment_name, COUNT(experiment_id) AS num_experiments
FROM
T AS t
LEFT JOIN Experiments USING (platform, experiment_name)
GROUP BY 1, 2;""").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.