Leetcode #2820: Election Results
In this guide, we solve Leetcode #2820 Election Results 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: Votes +-------------+---------+ | Column Name | Type | +-------------+---------+ | voter | varchar | | candidate | varchar | +-------------+---------+ (voter, candidate) is the primary key (combination of unique values) for this table. Each row of this table contains name of the voter and their candidate.
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 |
+-------------+---------+
| voter | varchar |
| candidate | varchar |
+-------------+---------+
(voter, candidate) is the primary key (combination of unique values) for this table.
Each row of this table contains name of the voter and their candidate.
Python Solution
import duckdb
import pandas as pd
# Pass input tables as keyword arguments matching the SQL table names.
def solution(**tables) -> pd.DataFrame:
con = duckdb.connect()
for name, df in tables.items():
con.register(name, df)
return con.execute("""WITH
T AS (
SELECT candidate, SUM(vote) AS tot
FROM
(
SELECT
candidate,
1 / (COUNT(candidate) OVER (PARTITION BY voter)) AS vote
FROM Votes
WHERE candidate IS NOT NULL
) AS t
GROUP BY 1
),
P AS (
SELECT
candidate,
RANK() OVER (ORDER BY tot DESC) AS rk
FROM T
)
SELECT candidate
FROM P
WHERE rk = 1
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.