207. Course Schedule

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 1:

• Input: numCourses = 2, prerequisites = [[1,0]]
• Output: true
• Explanation: There are a total of 2 courses to take. To take course 1 you should have finished course 0. So it is possible.

Example 2:

• Input: numCourses = 2, prerequisites = [[1,0],[0,1]]
• Output: false
• Explanation: There are a total of 2 courses to take. To take course 1 you should have finished course 0, and to take course 0 you should also have finished course 1. So it is impossible.

Constraints:

• 1 <= numCourses <= 2000
• 0 <= prerequisites.length <= 5000
• prerequisites[i].length == 2
• 0 <= ai, bi < numCourses
• All the pairs prerequisites[i] are unique.

Solution

class Solution:
    def canFinish(self, numCourses: int, prerequisites: list[list[int]]) -> bool:
        indegree = [0] * numCourses
        adj = [[] for x in range(numCourses)]
        for prereq in prerequisites:
            adj[prereq[1]].append(prereq[0])
            indegree[prereq[0]] += 1

        queue = []

        for i in range(numCourses):
            if indegree[i] == 0:
                queue.append(i)

        visited = 0
        while queue:
            node = queue.pop(0)
            visited += 1
            for neighbor in adj[node]:
                indegree[neighbor] -= 1
                if indegree[neighbor] == 0:
                    queue.append(neighbor)

        return numCourses == visited