Add Longest Common Subsequence in Ruby

archive/r/ruby/longest-common-subsequence.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+USAGE = 'Usage: please provide two lists in the format "1, 2, 3, 4, 5"'
+
+def usage!
+  warn USAGE
+  exit 1
+end
+
+def parse_list(str)
+  str.split(",").map { Integer(it.strip) }
+rescue ArgumentError, NoMethodError
+  usage!
+end
+
+def lcs(a, b)
+  n = a.size
+  m = b.size
+
+  dp = Array.new(n + 1) { Array.new(m + 1, 0) }
+
+  (1..n).each do |i|
+    (1..m).each do |j|
+      dp[i][j] = if a[i - 1] == b[j - 1]
+        dp[i - 1][j - 1] + 1
+      else
+        [dp[i - 1][j], dp[i][j - 1]].max
+      end
+    end
+  end
+
+  i, j = n, m
+  result = []
+
+  while i > 0 && j > 0
+    if a[i - 1] == b[j - 1]
+      result << a[i - 1]
+      i -= 1
+      j -= 1
+    elsif dp[i - 1][j] >= dp[i][j - 1]
+      i -= 1
+    else
+      j -= 1
+    end
+  end
+
+  result.reverse
+end
+
+a_str, b_str = ARGV
+usage! if a_str.nil? || b_str.nil? || a_str.strip.empty? || b_str.strip.empty?
+
+a = parse_list(a_str)
+b = parse_list(b_str)
+
+puts lcs(a, b).join(", ")