[LeetCode] 1059. All Paths from Source Lead to Destination

 Problem: https://leetcode.com/problems/all-paths-from-source-lead-to-destination/

We need to check if all paths start from 'source' can end up with the 'destination' and there is no cycle exists on the paths.

class Solution:
    def leadsToDestination(self, n: int, edges: List[List[int]], source: int, destination: int) -> bool:
        graph = defaultdict(set)
        
        for a, b in edges:
            graph[a].add(b)
            
        def dfs(a, visited):
            if a in visited:
                # detect a cycle
                return False
            
            if not graph[a]:
                # check if it ends up at 'destination'
                return a == destination
            
            visited.add(a)
            
            for b in graph[a]:
                if not dfs(b, visited):
                    return False
            
            visited.remove(a)
            return True
        
        return dfs(source, set())