1557. Minimum Number of Vertices to Reach all Nodes - Explanation

class Solution:
    def findSmallestSetOfVertices(self, n: int, edges: List[List[int]]) -> List[int]:
        adj = [[] for _ in range(n)]
        for u, v in edges:
            adj[u].append(v)

        res = set(range(n))
        visited = [False] * n

        def dfs(node):
            visited[node] = True
            for nei in adj[node]:
                if not visited[nei]:
                    dfs(nei)
                res.discard(nei)

        for i in range(n):
            if not visited[i]:
                dfs(i)
        return list(res)

class Solution:
    def findSmallestSetOfVertices(self, n: int, edges: List[List[int]]) -> List[int]:
        adj = [[] for _ in range(n)]
        for u, v in edges:
            adj[u].append(v)

        res = [True] * n
        visited = [False] * n
        stack = []

        for i in range(n):
            if not visited[i]:
                stack.append(i)
                while stack:
                    node = stack.pop()
                    if visited[node]:
                        continue
                    visited[node] = True
                    for nei in adj[node]:
                        if not visited[nei]:
                            stack.append(nei)
                        res[nei] = False

        return [i for i in range(n) if res[i]]

class Solution:
    def findSmallestSetOfVertices(self, n: int, edges: List[List[int]]) -> List[int]:
        incoming = collections.defaultdict(list)
        for src, dst in edges:
            incoming[dst].append(src)

        res = []
        for i in range(n):
            if not incoming[i]:
                res.append(i)
        return res

class Solution:
    def findSmallestSetOfVertices(self, n: int, edges: List[List[int]]) -> List[int]:
        indegree = [False] * n
        for src, dst in edges:
            indegree[dst] = True
        return [i for i in range(n) if not indegree[i]]