207. Course Schedule.py

'''
There are a total of numCourses courses you have to take, labeled from 0 to numCourses-1.

Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed as a pair: [0,1]

Given the total number of courses and a list of prerequisite pairs, is it possible for you to finish all courses?

 

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:

The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.
You may assume that there are no duplicate edges in the input prerequisites.
1 <= numCourses <= 10^5
'''

from queue import Queue
class Solution:
    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
        graph = {}
        indegree = {}
        
        for i in range(numCourses):
            graph[i] = []
            indegree[i] = 0

        for ed in prerequisites:
            graph[ed[1]].append(ed[0])
            indegree[ed[0]]+=1
        
        q = Queue()
        for k,v in indegree.items():
            if v==0:
                q.put(k)
        
        res = []
        while(q.qsize()):
            node = q.get()
            res.append(node)
            
            for nn in graph[node]:
                indegree[nn]-=1
                if indegree[nn]==0:
                    q.put(nn)
                    
        if len(res)==numCourses:
            return True
        else:
            return False