diff --git a/README.md b/README.md
index 1bce8690..faecd003 100644
--- a/README.md
+++ b/README.md
@@ -614,3 +614,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 701. Insert into a Binary Search Tree                           | [Link](https://leetcode.com/problems/insert-into-a-binary-search-tree/)                           | [Link](./lib/medium/701_insert_into_a_binary_search_tree.rb)                           | [Link](./test/medium/test_701_insert_into_a_binary_search_tree.rb)                           |
 | 707. Design Linked List                                         | [Link](https://leetcode.com/problems/design-linked-list/)                                         | [Link](./lib/medium/707_design_linked_list.rb)                                         | [Link](./test/medium/test_707_design_linked_list.rb)                                         |
 | 713. Subarray Product Less Than K                               | [Link](https://leetcode.com/problems/subarray-product-less-than-k/)                               | [Link](./lib/medium/713_subarray_product_less_than_k.rb)                               | [Link](./test/medium/test_713_subarray_product_less_than_k.rb)                               |
+| 720. Longest Word in Dictionary                                 | [Link](https://leetcode.com/problems/longest-word-in-dictionary/)                                 | [Link](./lib/medium/720_longest_word_in_dictionary.rb)                                 | [Link](./test/medium/test_720_longest_word_in_dictionary.rb)                                 |
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index 128c9822..d8504640 100644
--- a/leetcode-ruby.gemspec
+++ b/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 = '7.4.9'
+  s.version = '7.5.0'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/medium/720_longest_word_in_dictionary.rb b/lib/medium/720_longest_word_in_dictionary.rb
new file mode 100644
index 00000000..6993f139
--- /dev/null
+++ b/lib/medium/720_longest_word_in_dictionary.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+require 'set'
+
+# https://leetcode.com/problems/longest-word-in-dictionary/
+# @param {String[]} words
+# @return {String}
+def longest_word(words)
+  found_words = { '' => true }
+  words.sort.each { |word| found_words[word] = true if found_words[word[...-1]] }
+
+  found_words.keys.max_by(&:size)
+end
diff --git a/test/medium/test_720_longest_word_in_dictionary.rb b/test/medium/test_720_longest_word_in_dictionary.rb
new file mode 100644
index 00000000..ac047eae
--- /dev/null
+++ b/test/medium/test_720_longest_word_in_dictionary.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/medium/720_longest_word_in_dictionary'
+require 'minitest/autorun'
+
+class LongestWordInDictionaryTest < ::Minitest::Test
+  def test_default_one
+    assert_equal(
+      'world',
+      longest_word(
+        %w[w wo wor worl world]
+      )
+    )
+  end
+
+  def test_default_two
+    assert_equal(
+      'apple',
+      longest_word(
+        %w[a banana app appl ap apply apple]
+      )
+    )
+  end
+end