0802-find-eventual-safe-states.rb

# frozen_string_literal: true

# 802. Find Eventual Safe States
# Medium
# https://leetcode.com/problems/find-eventual-safe-states

=begin
There is a directed graph of n nodes with each node labeled from 0 to n - 1. The graph is represented by a 0-indexed 2D integer array graph where graph[i] is an integer array of nodes adjacent to node i, meaning there is an edge from node i to each node in graph[i].
A node is a terminal node if there are no outgoing edges. A node is a safe node if every possible path starting from that node leads to a terminal node (or another safe node).
Return an array containing all the safe nodes of the graph. The answer should be sorted in ascending order.

Example 1:
Input: graph = [[1,2],[2,3],[5],[0],[5],[],[]]
Output: [2,4,5,6]
Explanation: The given graph is shown above.
Nodes 5 and 6 are terminal nodes as there are no outgoing edges from either of them.
Every path starting at nodes 2, 4, 5, and 6 all lead to either node 5 or 6.

Example 2:
Input: graph = [[1,2,3,4],[1,2],[3,4],[0,4],[]]
Output: [4]
Explanation:
Only node 4 is a terminal node, and every path starting at node 4 leads to node 4.

Constraints:
* n == graph.length
* 1 <= n <= 104
* 0 <= graph[i].length <= n
* 0 <= graph[i][j] <= n - 1
* graph[i] is sorted in a strictly increasing order.
* The graph may contain self-loops.
* The number of edges in the graph will be in the range [1, 4 * 104].
=end

# @param {Integer[][]} graph
# @return {Integer[]}
def eventual_safe_nodes(graph)
  node_states = Array.new(graph.size, :unvisited)
  graph.each_index { |i| dfs(i, graph, node_states) }
  node_states.each_index.select { |i| node_states[i] == :safe }.sort
end

def dfs(node, graph, node_states)
  return false if node_states[node] == :unsafe
  return true if node_states[node] == :safe

  node_states[node] = :unsafe
  graph[node].each do |neighbour|
    return false unless dfs(neighbour, graph, node_states)
  end

  node_states[node] = :safe
  true
end