207. Course Schedule

Medium

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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.

• For example, the pair [0, 1], indicates that to take course 0 you have to first take course 1.

Return true if you can finish all courses. Otherwise, return false.

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 <= 105

• 0 <= prerequisites.length <= 5000

• prerequisites[i].length == 2

• 0 <= ai, bi < numCourses

• All the pairs prerequisites[i] are unique.

from collections import defaultdict
class Solution:
    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
        degree = [0]*numCourses
        d = defaultdict(list)
        q = deque()
        ans = []
        for prev,nxt in prerequisites:
            degree[nxt] += 1
            d[prev].append(nxt)
        for index in range(numCourses):
            if degree[index] == 0:
                q.append(index)
        while q:
            course = q.pop()
            ans.append(course)
            for adjCourse in d[course]:
                degree[adjCourse] -= 1
                if degree[adjCourse] == 0:
                    q.append(adjCourse)
        return True if len(ans) == numCourses else False