2025-03-20 v. 9.0.4: added "2131. Longest Palindrome by Concatenating Two Letter Words"

README.md

@@ -738,6 +738,7 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 2121. Intervals Between Identical Elements                                          | [Link](https://leetcode.com/problems/intervals-between-identical-elements/)                                          | [Link](./lib/medium/2121_intervals_between_identical_elements.rb)                                          | [Link](./test/medium/test_2121_intervals_between_identical_elements.rb)                                          |
 | 2125. Number of Laser Beams in a Bank                                               | [Link](https://leetcode.com/problems/number-of-laser-beams-in-a-bank/)                                               | [Link](./lib/medium/2125_number_of_laser_beams_in_a_bank.rb)                                               | [Link](./test/medium/test_2125_number_of_laser_beams_in_a_bank.rb)                                               |
 | 2130. Maximum Twin Sum of a Linked List                                             | [Link](https://leetcode.com/problems/maximum-twin-sum-of-a-linked-list/)                                             | [Link](./lib/medium/2130_maximum_twin_sum_of_a_linked_list.rb)                                             | [Link](./test/medium/test_2130_maximum_twin_sum_of_a_linked_list.rb)                                             |
+| 2131. Longest Palindrome by Concatenating Two Letter Words                          | [Link](https://leetcode.com/problems/longest-palindrome-by-concatenating-two-letter-words/)                          | [Link](./lib/medium/2131_longest_palindrome_by_concatenating_two_letter_words.rb)                          | [Link](./test/medium/test_2131_longest_palindrome_by_concatenating_two_letter_words.rb)                          |
 | 2425. Bitwise XOR of All Pairings                                                   | [Link](https://leetcode.com/problems/bitwise-xor-of-all-pairings/)                                                   | [Link](./lib/medium/2425_bitwise_xor_of_all_pairings.rb)                                                   | [Link](./test/medium/test_2425_bitwise_xor_of_all_pairings.rb)                                                   |
 | 2429. Minimize XOR                                                                  | [Link](https://leetcode.com/problems/minimize-xor/)                                                                  | [Link](./lib/medium/2429_minimize_xor.rb)                                                                  | [Link](./test/medium/test_2429_minimize_xor.rb)                                                                  |
 | 2657. Find the Prefix Common Array of Two Arrays                                    | [Link](https://leetcode.com/problems/find-the-prefix-common-array-of-two-arrays/)                                    | [Link](./lib/medium/2657_find_the_prefix_common_array_of_two_arrays.rb)                                    | [Link](./test/medium/test_2657_find_the_prefix_common_array_of_two_arrays.rb)                                    |

leetcode-ruby.gemspec

@@ -5,7 +5,7 @@ require 'English'
 ::Gem::Specification.new do |s|
   s.required_ruby_version = '>= 3.0'
   s.name = 'leetcode-ruby'
-  s.version = '9.0.3'
+  s.version = '9.0.4'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'

lib/medium/2131_longest_palindrome_by_concatenating_two_letter_words.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/longest-palindrome-by-concatenating-two-letter-words/
+# @param {String[]} words
+# @return {Integer}
+def longest_palindrome2131(words)
+  freq = ::Hash.new(0)
+  words.each { |word| freq[word] += 1 }
+
+  total = 0
+  processed = ::Set.new
+
+  freq.each_key do |word|
+    rev = word.reverse
+
+    next if word == rev
+
+    next if processed.include?(word)
+
+    processed.add(word)
+    processed.add(rev)
+
+    current_count = freq[word]
+    rev_count = freq[rev]
+    min_pairs = [current_count, rev_count].min
+    total += min_pairs * 4
+  end
+
+  has_odd_count = false
+
+  freq.each do |word, count|
+    next unless word == word.reverse
+
+    pairs = count / 2
+    total += pairs * 4
+
+    has_odd_count ||= count.odd?
+  end
+
+  total += 2 if has_odd_count
+
+  total
+end

test/medium/test_2131_longest_palindrome_by_concatenating_two_letter_words.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/medium/2131_longest_palindrome_by_concatenating_two_letter_words'
+require 'minitest/autorun'
+
+class LongestPalindromeByConcatenatingTwoLetterWordsTest < ::Minitest::Test
+  def test_default_one
+    assert_equal(
+      6,
+      longest_palindrome2131(
+        %w[lc cl gg]
+      )
+    )
+  end
+
+  def test_default_two
+    assert_equal(
+      8,
+      longest_palindrome2131(
+        %w[ab ty yt lc cl ab]
+      )
+    )
+  end
+
+  def test_default_three
+    assert_equal(
+      2,
+      longest_palindrome2131(
+        %w[cc ll xx]
+      )
+    )
+  end
+end