Leetcode #176: Second Highest Salary
In this guide, we solve Leetcode #176 Second Highest Salary 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: Employee +-------------+------+ | Column Name | Type | +-------------+------+ | id | int | | salary | int | +-------------+------+ id is the primary key (column with unique values) for this table. Each row of this table contains information about the salary of an employee.
Quick Facts
- Difficulty: Medium
- 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 |
+-------------+------+
| id | int |
| salary | int |
+-------------+------+
id is the primary key (column with unique values) for this table.
Each row of this table contains information about the salary of an employee.
Python Solution
import pandas as pd
def second_highest_salary(employee: pd.DataFrame) -> pd.DataFrame:
# Drop any duplicate salary values to avoid counting duplicates as separate salary ranks
unique_salaries = employee["salary"].drop_duplicates()
# Sort the unique salaries in descending order and get the second highest salary
second_highest = (
unique_salaries.nlargest(2).iloc[-1] if len(unique_salaries) >= 2 else None
)
# If the second highest salary doesn't exist (e.g., there are fewer than two unique salaries), return None
if second_highest is None:
return pd.DataFrame({"SecondHighestSalary": [None]})
# Create a DataFrame with the second highest salary
result_df = pd.DataFrame({"SecondHighestSalary": [second_highest]})
return result_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.