All Ancestors of a Node in a Directed Acyclic Graph — LeetCode 2192 Python Solution

The problem

You are given a positive integer n representing the number of nodes of a Directed Acyclic Graph (DAG). The nodes are numbered from 0 to n - 1 (inclusive).

Example

Input

n = 8, edgeList = [[0,3],[0,4],[1,3],[2,4],[2,7],[3,5],[3,6],[3,7],[4,6]]

Output

[[],[],[],[0,1],[0,2],[0,1,3],[0,1,2,3,4],[0,1,2,3]]

Explanation

The above diagram represents the input graph.

Python solution

class Solution:
    def getAncestors(self, n: int, edges: List[List[int]]) -> List[List[int]]:
        def bfs(s: int):
            q = deque([s])
            vis = {s}
            while q:
                i = q.popleft()
                for j in g[i]:
                    if j not in vis:
                        vis.add(j)
                        q.append(j)
                        ans[j].append(s)

        g = defaultdict(list)
        for u, v in edges:
            g[u].append(v)
        ans = [[] for _ in range(n)]
        for i in range(n):
            bfs(i)
        return ans

Complexity

Pattern: Topological Sort

Order a set of tasks so that every dependency comes before the thing that needs it. LeetCode 2192. All Ancestors of a Node in a Directed Acyclic Graph is filed here because LeetCode tags it Topological Sort, which is the vocabulary this hub collects.

The topological sort guide has the Python template for the pattern and the 32 LeetCode problems that use it.

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