LeetCode 574: Winning Candidate

Problem Restatement

We are given two tables.

Candidate:

Column	Type	Meaning
id	int	Candidate id
name	varchar	Candidate name

Vote:

Column	Type	Meaning
id	int	Vote id
candidateId	int	The candidate who received this vote

We need to report the name of the candidate who received the largest number of votes.

The test cases guarantee exactly one winner. The result table can be returned in any order because there is only one winning row.

Input and Output

Item	Meaning
Input	Candidate and Vote tables
Output	Name of the winning candidate
Winner	Candidate with the largest vote count
Tie	The problem guarantees one winner

Expected output column:

name

Example

Candidate table:

id	name
1	A
2	B
3	C
4	D
5	E

Vote table:

id	candidateId
1	2
2	4
3	3
4	2
5	5

Vote counts:

candidateId	Candidate	Votes
2	B	2
3	C	1
4	D	1
5	E	1

Candidate B has the most votes.

Output:

name
B

First Thought: Count Votes Per Candidate

Each row in Vote represents one vote.

So we can group votes by candidateId:

GROUP BY candidateId

Then count how many rows are in each group:

COUNT(*)

The candidate with the largest count is the winner.

After finding the winning candidateId, we join back to the Candidate table to get the candidate's name.

Key Insight

The Vote table gives counts.

The Candidate table gives names.

So the query naturally has two steps:

1. Aggregate votes to find the winning candidateId.
2. Join that id to Candidate to return the name.

Because the problem guarantees one winner, we can sort by vote count descending and use:

LIMIT 1

Algorithm

1. Group the Vote table by candidateId.
2. Count votes for each candidate.
3. Sort candidates by vote count descending.
4. Keep the first candidate.
5. Join that candidate id with Candidate.id.
6. Return Candidate.name.

Correctness

Grouping the Vote table by candidateId puts all votes for the same candidate into one group.

COUNT(*) gives the exact number of votes received by that candidate.

Sorting the groups by COUNT(*) DESC places the candidate with the largest vote count first.

Since the problem guarantees exactly one winner, LIMIT 1 selects the winning candidate id.

Joining this id to Candidate.id retrieves the corresponding candidate row. Selecting name returns exactly the winning candidate's name.

Complexity

Let v be the number of rows in Vote, and let c be the number of rows in Candidate.

Metric	Value	Why
Time	O(v log v) or better	The database groups and sorts vote counts
Space	O(c)	The grouping step stores one count per candidate with votes

The exact cost depends on the database engine and indexes.

Implementation

SELECT c.name
FROM Candidate AS c
JOIN (
    SELECT candidateId
    FROM Vote
    GROUP BY candidateId
    ORDER BY COUNT(*) DESC
    LIMIT 1
) AS winner
    ON c.id = winner.candidateId;

Code Explanation

The subquery finds the winning candidate id:

SELECT candidateId
FROM Vote
GROUP BY candidateId
ORDER BY COUNT(*) DESC
LIMIT 1

GROUP BY candidateId creates one group per candidate.

COUNT(*) counts that candidate's votes.

ORDER BY COUNT(*) DESC puts the highest vote count first.

LIMIT 1 keeps only the winner.

Then the outer query joins the winner to the candidate table:

ON c.id = winner.candidateId

Finally, it returns:

SELECT c.name

Join-First Version

We can also join first, then group by candidate.

SELECT c.name
FROM Candidate AS c
JOIN Vote AS v
    ON c.id = v.candidateId
GROUP BY c.id, c.name
ORDER BY COUNT(*) DESC
LIMIT 1;

This version is shorter and also correct.

It joins each vote to its candidate, groups by candidate, sorts by vote count, and returns the top candidate.

Testing

Create sample data:

CREATE TABLE Candidate (
    id INT PRIMARY KEY,
    name VARCHAR(255)
);

CREATE TABLE Vote (
    id INT PRIMARY KEY,
    candidateId INT
);

INSERT INTO Candidate (id, name) VALUES
(1, 'A'),
(2, 'B'),
(3, 'C'),
(4, 'D'),
(5, 'E');

INSERT INTO Vote (id, candidateId) VALUES
(1, 2),
(2, 4),
(3, 3),
(4, 2),
(5, 5);

Expected result:

name
B

Additional cases:

Case	Expected behavior
One candidate has all votes	Return that candidate
Several candidates have votes	Return the one with highest count
Candidate has no votes	Does not affect the winner
Exactly one winning candidate	LIMIT 1 is safe under the problem guarantee