802. Find Eventual Safe States

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https://leetcode.com/problems/find-eventual-safe-states/

In a directed graph, we start at some node and every turn, walk along a directed edge of the graph.  If we reach a node that is terminal (that is, it has no outgoing directed edges), we stop.
Now, say our starting node is eventually safe if and only if we must eventually walk to a terminal node.  More specifically, there exists a natural number K so that for any choice of where to walk, we must have stopped at a terminal node in less than K steps.
Which nodes are eventually safe?  Return them as an array in sorted order.
The directed graph has N nodes with labels 0, 1, ..., N-1, where N is the length of graph.  The graph is given in the following form: graph[i] is a list of labels j such that (i, j) is a directed edge of the graph.
Example:
Input: graph = [[1,2],[2,3],[5],[0],[5],[],[]]
Output: [2,4,5,6]
Here is a diagram of the above graph.
Illustration of graph
Note:
  • graph will have length at most 10000.
  • The number of edges in the graph will not exceed 32000.
  • Each graph[i] will be a sorted list of different integers, chosen within the range [0, graph.length - 1].
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Intuition
Eventual safe states are nodes with no outgoing Edges
Any node pointing into it is potentially a candidate

Initialize a array of outDegree
Identify the nodes with outDegree 0

Nodes pointing into nodes with outDegree 0 are potential candidates
We need to quickly lookup nodes pointing into current node
Create another graph to hold this info
Node, Set of Nodes pointing into it

Start with node with outDegree 0
Get nodes pointing into it
Since starting node is part of answer, reduce out Degree of nodes pointing into it by 1
if their out degree become 0, they are part of ans

Continue this till the q of nodes with out Degree 0 is empty

Since answer is expected in sorted order
Scan the outDegree array linear to avoid cost of sorting
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Time - O(|V|)
Space - O(|V + E|)
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