Leetcode 207 Course Schedule

Problem Statement

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.

DFS: Recurisve

from collections import defaultdict
class Solution:
    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
        adj = defaultdict(set)
        seen = set()

        for a, b in prerequisites:
            adj[b].add(a)

        def cycle_found(adj, u, seen):
            if u in seen:
                return True

            seen.add(u)

            for v in adj[u]:
                if cycle_found(adj, v, seen):
                    return True
            
            adj[u] = []

            seen.remove(u)

            return False

        for i in range(numCourses):
            if cycle_fount(adj, i, seen):
                return False
        
        return True

Leetcode 207 Course Schedule