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71 lines
1.2 KiB
Ruby
71 lines
1.2 KiB
Ruby
require 'set'
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# Finding a directed cycle in a directed graph.
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# The graph is represented as an adjacency list of Integers,
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# see examples in test/directed_cycle_test.rb
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class DirectedCycle
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class Digraph
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attr_accessor :adj, :size
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def initialize(size)
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@size = size
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@adj = Array.new(size) { Set.new }
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end
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def add_edge(v, w)
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@adj[v].add(w)
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end
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end
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def initialize(graph)
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@graph = graph
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@cycle = []
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@visited = Array.new(graph.size, false)
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@on_stack = Array.new(graph.size, false)
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@edge_to = Array.new(graph.size, nil)
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graph.size.times do |v|
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dfs(v) unless @visited[v]
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end
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end
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def has_cycle?
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@cycle.any?
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end
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def cycle
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@cycle
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end
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private
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def dfs(v)
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@visited[v] = true
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@on_stack[v] = true
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@graph.adj[v].each do |w|
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return if has_cycle?
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if @visited[w]
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track_cycle(v, w) if @on_stack[w]
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else
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@edge_to[w] = v
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dfs(w)
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end
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end
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@on_stack[v] = false
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end
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def track_cycle(from, to)
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v = from
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while v != to
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@cycle.push(v)
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v = @edge_to[v]
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end
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@cycle.push(to)
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@cycle.push(from)
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@cycle.reverse!
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end
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end
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