Leetcode 207. Course Schedule. Python

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.

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

class Solution:
    def canFinish(self, n: int, preq: List[List[int]]) -> bool:
        
        hm = { i: [] for i in range(n)}
        vs = set()
        
        for c,p in preq :
            hm[c].append(p)
            
        def dfs(c):
            if c in vs:
                return False
            if hm[c] == []:
                return True
            
            vs.add(c)
            
            for p in hm[c]:
                if not dfs(p): return False
                
            vs.remove(c)
            hm[c] = []
            
            return True
        
        for c in range(n):
            if not dfs(c): 
                return False
        return True

Explaination :

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