Course Schedule II — LeetCode 210 Python Solution

The problem

There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You are given an array prerequisites where prerequisites[i] = [ai, bi] indicates that you must take course bi first if you want to take course ai.

Example

Input

numCourses = 2, prerequisites = [[1,0]]

Output

[0,1]

Explanation

There are a total of 2 courses to take. To take course 1 you should have finished course 0. So the correct course order is [0,1].

Python solution

class Solution:
    def findOrder(self, numCourses: int, prerequisites: List[List[int]]) -> List[int]:
        g = defaultdict(list)
        indeg = [0] * numCourses
        for a, b in prerequisites:
            g[b].append(a)
            indeg[a] += 1
        ans = []
        q = deque(i for i, x in enumerate(indeg) if x == 0)
        while q:
            i = q.popleft()
            ans.append(i)
            for j in g[i]:
                indeg[j] -= 1
                if indeg[j] == 0:
                    q.append(j)
        return ans if len(ans) == numCourses else []

Complexity

Pattern: Topological Sort

Order a set of tasks so that every dependency comes before the thing that needs it. LeetCode 210. Course Schedule II is filed here because LeetCode tags it Topological Sort, which is the vocabulary this hub collects.

The topological sort guide has the Python template for the pattern and the 32 LeetCode problems that use it.

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