LeetCode 210: Course Schedule II

Meta Data

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Difficulty: medium First Attempt: 2026-04-08 Source: Day 1 learning note

Study Context

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This article is rebuilt from the exact LeetCode section in the learning note. I kept the note’s repair points, comparison points, and common mistakes, while removing unrelated non-LeetCode material from the same day.

Learning Note Extract

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LC 210 — Course Schedule II (Topological Sort / Return Order)

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• Pattern: Same as LC 207 but return the actual ordering
• Key insight: The order nodes are popped from the queue IS the topological order
• Difference from LC 207: Append each popped node to result list. If len(result) == numCourses → valid order exists.

Organized Notes

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This is the order-returning version of LC 207. The graph and indegree construction are identical: an edge goes from prerequisite to course. The difference is that every popped node is appended to order, and the result is valid only if the order covers every course. If a cycle remains, some indegrees never drop to zero, so returning the partial order would be wrong.

Clean Solution

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The note above captures the reasoning and the mistakes to avoid. The implementation below is the version I would submit.

from collections import deque
from typing import List

class Solution:
    def findOrder(self, numCourses: int, prerequisites: List[List[int]]) -> List[int]:
        graph = [[] for _ in range(numCourses)]
        indeg = [0] * numCourses
        for course, pre in prerequisites:
            graph[pre].append(course)
            indeg[course] += 1

        q = deque(i for i in range(numCourses) if indeg[i] == 0)
        order = []
        while q:
            node = q.popleft()
            order.append(node)
            for nei in graph[node]:
                indeg[nei] -= 1
                if indeg[nei] == 0:
                    q.append(nei)

        return order if len(order) == numCourses else []

Complexity

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Time O(V+E), Space O(V+E).

Mistakes To Watch

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• Returning partial order when a cycle remains.
• Confusing prerequisite edge direction.

Final Interview Explanation

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I would use the same Kahn topo process as Course Schedule, but append each popped course to an order list. The order is valid only if it contains all courses; otherwise a cycle blocked some courses, and the correct return value is an empty list.