Leetcode #580: Count Student Number in Departments
In this guide, we solve Leetcode #580 Count Student Number in Departments 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: Student +--------------+---------+ | Column Name | Type | +--------------+---------+ | student_id | int | | student_name | varchar | | gender | varchar | | dept_id | int | +--------------+---------+ student_id is the primary key (column with unique values) for this table. dept_id is a foreign key (reference column) to dept_id in the Department tables.
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 |
+--------------+---------+
| student_id | int |
| student_name | varchar |
| gender | varchar |
| dept_id | int |
+--------------+---------+
student_id is the primary key (column with unique values) for this table.
dept_id is a foreign key (reference column) to dept_id in the Department tables.
Each row of this table indicates the name of a student, their gender, and the id of their department.
Python Solution
import pandas as pd
def count_students(student: pd.DataFrame, department: pd.DataFrame) -> pd.DataFrame:
counts = student.groupby('dept_id')['student_id'].count().reset_index(name='student_number')
res = department.merge(counts, on='dept_id', how='left')
res['student_number'] = res['student_number'].fillna(0).astype(int)
return res[['dept_name', 'student_number']]
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.