How do you solve Employees Whose Manager Left the Company on LeetCode?

Employees Whose Manager Left the Company (LeetCode #1978) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: Understanding how to check for non-existent references (managers) is crucial in relational databases.

What is the time complexity of Employees Whose Manager Left the Company?

The optimal solution for Employees Whose Manager Left the Company runs in O(n) time and uses O(n) space. The complexity is O(n) because we are scanning through the Employees table once and performing a join operation, which is efficient.

What is the brute force solution for Employees Whose Manager Left the Company?

The brute force approach for Employees Whose Manager Left the Company is Brute Force, with O(n²) time complexity and O(1) space complexity. In this approach, we will check each employee to see if their salary is less than $30,000 and then verify if their manager exists in the Employees table. It's straightforward but inefficient. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Employees Whose Manager Left the Company?

Employees Whose Manager Left the Company uses the following concepts: Database. The recommended patterns to study are: JOIN operations, Subqueries, Filtering with conditions.

What are the interview tips for Employees Whose Manager Left the Company?

Always clarify the requirements and constraints of the problem before jumping into coding. Think about edge cases, such as employees without managers or all employees having salaries above the threshold. Practice writing SQL queries on sample data to get comfortable with JOINs and subqueries.

What are common mistakes when solving Employees Whose Manager Left the Company?

Not considering employees with null manager_ids when filtering. Overlooking the need to check if the manager_id exists in the Employees table.

#1978

Employees Whose Manager Left the Company

Easy

DatabaseJOIN operationsSubqueriesFiltering with conditions

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

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Intuition

Time O(n)Space O(n)

This approach uses a single query to find employees with a salary less than $30,000 and checks for their managers in a more efficient way by using a LEFT JOIN.

⚙️

Algorithm

3 steps

1Step 1: Perform a LEFT JOIN on the Employees table to find employees with a salary less than $30,000 and their corresponding manager.
2Step 2: Filter the results to include only those employees whose manager_id is not found in the Employees table (i.e., the manager has left).
3Step 3: Select the employee_id of these filtered employees.

solution.py1 lines

1SELECT e.employee_id FROM Employees e LEFT JOIN Employees m ON e.manager_id = m.employee_id WHERE e.salary < 30000 AND m.employee_id IS NULL;

ℹ

Complexity note: The complexity is O(n) because we are scanning through the Employees table once and performing a join operation, which is efficient.

1Understanding how to check for non-existent references (managers) is crucial in relational databases.
2Using JOINs can significantly reduce the number of queries and improve performance.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.