CourseSchedule.py

'''
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.
'''

class Solution:
    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
        preMap = {i:[] for i in range(numCourses)}
        
        visit = set()
        
        for crs, pre in prerequisites:
            preMap[crs].append(pre)
            
        def dfs(crs):
            if crs in visit:
                return False
            if preMap[crs] == []:
                return True
            visit.add(crs)
            for pre in preMap[crs]:
                if not dfs(pre):
                    return False
            visit.remove(crs)
            preMap[crs] = []
            return True
            
        
        
        
        for crs in range(numCourses):
            if not dfs(crs):
                return False
        return True

'''
Runtime: 92 ms, faster than 90.68% of Python3 online submissions for Course Schedule.
Memory Usage: 17.1 MB, less than 32.47% of Python3 online submissions for Course Schedule.
'''