Leetcode #1978: Employees Whose Manager Left the Company
In this guide, we solve Leetcode #1978 Employees Whose Manager Left the Company 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: Employees +-------------+----------+ | Column Name | Type | +-------------+----------+ | employee_id | int | | name | varchar | | manager_id | int | | salary | int | +-------------+----------+ In SQL, employee_id is the primary key for this table. This table contains information about the employees, their salary, and the ID of their manager.
Quick Facts
- Difficulty: Easy
- Premium: No
- 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 |
+-------------+----------+
| employee_id | int |
| name | varchar |
| manager_id | int |
| salary | int |
+-------------+----------+
In SQL, employee_id is the primary key for this table.
This table contains information about the employees, their salary, and the ID of their manager. Some employees do not have a manager (manager_id is null).
Python Solution
import duckdb
import pandas as pd
def solution(employees: pd.DataFrame) -> pd.DataFrame:
con = duckdb.connect()
con.register("Employees", employees)
return con.execute("""SELECT e1.employee_id
FROM
Employees AS e1
LEFT JOIN Employees AS e2 ON e1.manager_id = e2.employee_id
WHERE e1.salary < 30000 AND e1.manager_id IS NOT NULL AND e2.employee_id IS NULL
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.