1557-minimum-number-of-vertices-to-reach-all-nodes.rb

# frozen_string_literal: true

# 1557. Minimum Number of Vertices to Reach All Nodes
# https://leetcode.com/problems/minimum-number-of-vertices-to-reach-all-nodes
# Medium

=begin
Given a directed acyclic graph, with n vertices numbered from 0 to n-1, and an array edges where edges[i] = [fromi, toi] represents a directed edge from node fromi to node toi.

Find the smallest set of vertices from which all nodes in the graph are reachable. It's guaranteed that a unique solution exists.

Notice that you can return the vertices in any order.

Example 1:
Input: n = 6, edges = [[0,1],[0,2],[2,5],[3,4],[4,2]]
Output: [0,3]
Explanation: It's not possible to reach all the nodes from a single vertex. From 0 we can reach [0,1,2,5]. From 3 we can reach [3,4,2,5]. So we output [0,3].

Example 2:
Input: n = 5, edges = [[0,1],[2,1],[3,1],[1,4],[2,4]]
Output: [0,2,3]
Explanation: Notice that vertices 0, 3 and 2 are not reachable from any other node, so we must include them. Also any of these vertices can reach nodes 1 and 4.

Constraints:
2 <= n <= 10^5
1 <= edges.length <= min(10^5, n * (n - 1) / 2)
edges[i].length == 2
0 <= fromi, toi < n
All pairs (fromi, toi) are distinct.
=end

# @param {Integer} n
# @param {Integer[][]} edges
# @return {Integer[]}
def find_smallest_set_of_vertices(n, edges)
  (0...n).to_a - edges.map(&:last)
end

# **************** #
#       TEST       #
# **************** #

require "test/unit"

class Test_find_smallest_set_of_vertices < Test::Unit::TestCase
  def test_
    assert_equal [0, 3], find_smallest_set_of_vertices(6, [[0, 1], [0, 2], [2, 5], [3, 4], [4, 2]])
    assert_equal [0, 2, 3], find_smallest_set_of_vertices(5, [[0, 1], [2, 1], [3, 1], [1, 4], [2, 4]])
  end
end