Add Minimum Spanning Tree in Ruby (#5946)

Author: pascalecu

archive/r/ruby/minimum-spanning-tree.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+USAGE = "Usage: please provide a comma-separated list of integers"
+
+def usage!
+  warn USAGE
+  exit 1
+end
+
+def parse_input
+  raw = ARGV.first
+  usage! if raw.nil? || raw.strip.empty?
+
+  nums = raw.split(",").map { Integer(it.strip) }
+  n = Integer(Math.sqrt(nums.length))
+  usage! unless n * n == nums.length
+
+  Array.new(n) { |i| nums[i * n, n] }
+rescue ArgumentError, NoMethodError
+  usage!
+end
+
+def find(parent, x)
+  (parent[x] == x) ? x : (parent[x] = find(parent, parent[x]))
+end
+
+def union(parent, rank, a, b)
+  a = find(parent, a)
+  b = find(parent, b)
+  return if a == b
+
+  if rank[a] < rank[b]
+    parent[a] = b
+  elsif rank[a] > rank[b]
+    parent[b] = a
+  else
+    parent[b] = a
+    rank[a] += 1
+  end
+end
+
+def mst_weight(matrix)
+  n = matrix.size
+
+  edges =
+    (0...n).flat_map do |i|
+      (i + 1...n).map do |j|
+        w = matrix[i][j]
+        w.positive? ? [w, i, j] : nil
+      end
+    end.compact
+
+  edges.sort_by!(&:first)
+
+  parent = (0...n).to_a
+  rank = Array.new(n, 0)
+
+  edges.reduce(0) do |total, (w, u, v)|
+    next total if find(parent, u) == find(parent, v)
+
+    union(parent, rank, u, v)
+    total + w
+  end
+end
+
+matrix = parse_input
+puts mst_weight(matrix)