LeetCode 596: Classes With at Least 5 Students

Meta Data

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Difficulty: easy First Attempt: 2021-10-10

• Submissions: Wrong Answer ×3 → Runtime Error → Wrong Answer → Accepted

Approach

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As in LC 182 this filters groups (SQL Template 4): GROUP BY class with HAVING ... >= 5. Count distinct students, COUNT(DISTINCT student), so repeats do not count twice. The current problem makes (student, class) the primary key, so COUNT(*) also works, but DISTINCT is safer.

My Attempts

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This took 6 submissions, all about duplicate rows:

• The first versions used COUNT(class), and the 2021 test data listed the same student in the same class more than once, double counting them; attempt 1 also used SELECT *.
• Trying FROM distinct(courses) to deduplicate was invalid syntax (RE).
• Computing COUNT(DISTINCT student) in a subquery and filtering >= 5 outside passed; putting it straight into HAVING works without the subquery.

Solution

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SELECT class
FROM Courses
GROUP BY class
HAVING COUNT(DISTINCT student) >= 5;

Complexity

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Grouping is O(n) or O(n log n).

Mistakes to Avoid

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• SELECT * with GROUP BY fails in strict mode; select only grouped columns.
• Count distinct students with COUNT(DISTINCT student).

Interview Explanation

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I group by class and keep classes with at least five distinct students via HAVING, using COUNT DISTINCT so repeated rows do not count twice.