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Abhishek Chaudhary

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# All Ancestors of a Node in a Directed Acyclic Graph

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).

You are also given a 2D integer array `edges`, where `edges[i] = [fromi, toi]` denotes that there is a unidirectional edge from `fromi` to `toi` in the graph.

Return a list `answer`, where `answer[i]` is the list of ancestors of the `ith` node, sorted in ascending order.

A node `u` is an ancestor of another node `v` if `u` can reach `v` via a set of edges.

Example 1:

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.

• Nodes 0, 1, and 2 do not have any ancestors.
• Node 3 has two ancestors 0 and 1.
• Node 4 has two ancestors 0 and 2.
• Node 5 has three ancestors 0, 1, and 3.
• Node 6 has five ancestors 0, 1, 2, 3, and 4.
• Node 7 has four ancestors 0, 1, 2, and 3.

Example 2:

Input: n = 5, edgeList = [[0,1],[0,2],[0,3],[0,4],[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
Output: [[],[0],[0,1],[0,1,2],[0,1,2,3]]
Explanation:
The above diagram represents the input graph.

• Node 0 does not have any ancestor.
• Node 1 has one ancestor 0.
• Node 2 has two ancestors 0 and 1.
• Node 3 has three ancestors 0, 1, and 2.
• Node 4 has four ancestors 0, 1, 2, and 3.

Constraints:

• `1 <= n <= 1000`
• `0 <= edges.length <= min(2000, n * (n - 1) / 2)`
• `edges[i].length == 2`
• `0 <= fromi, toi <= n - 1`
• `fromi != toi`
• There are no duplicate edges.
• The graph is directed and acyclic.

SOLUTION:

``````class Solution:
def getAncestors(self, n: int, edges: List[List[int]]) -> List[List[int]]:
graph = {}
for a, b in edges:
graph[b] = graph.get(b, []) + [a]
op = [[] for i in range(n)]
for a in graph:
visited = set()
paths = [a]
while len(paths) > 0:
curr = paths.pop()
for b in graph.get(curr, []):
if b not in visited: