diff --git a/README.md b/README.md
index 5d822219..de2edd52 100644
--- a/README.md
+++ b/README.md
@@ -577,3 +577,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 445. Add Two Numbers II                                         | [Link](https://leetcode.com/problems/add-two-numbers-ii/)                                         | [Link](./lib/medium/445_add_two_numbers_ii.rb)                                         | [Link](./test/medium/test_445_add_two_numbers_ii.rb)                                         |
 | 449. Serialize and Deserialize BST                              | [Link](https://leetcode.com/problems/serialize-and-deserialize-bst/)                              | [Link](./lib/medium/449_serialize_and_deserialize_bst.rb)                              | [Link](./test/medium/test_449_serialize_and_deserialize_bst.rb)                              |
 | 450. Delete Node in a BST                                       | [Link](https://leetcode.com/problems/delete-node-in-a-bst/)                                       | [Link](./lib/medium/450_delete_node_in_a_bst.rb)                                       | [Link](./test/medium/test_450_delete_node_in_a_bst.rb)                                       |
+| 451. Sort Characters By Frequency                               | [Link](https://leetcode.com/problems/sort-characters-by-frequency/)                               | [Link](./lib/medium/451_sort_characters_by_frequency.rb)                               | [Link](./test/medium/test_451_sort_characters_by_frequency.rb)                               |
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index 9539054a..63106fe8 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 = '7.1.1'
+  s.version = '7.1.2'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/medium/451_sort_characters_by_frequency.rb b/lib/medium/451_sort_characters_by_frequency.rb
new file mode 100644
index 00000000..5d825bbd
--- /dev/null
+++ b/lib/medium/451_sort_characters_by_frequency.rb
@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/sort-characters-by-frequency/
+# @param {String} s
+# @return {String}
+def frequency_sort451(s)
+  arr = s.split('').sort
+  h = ::Hash.new { |v, k| v[k] = '' }
+  p1 = 0
+  p2 = 0
+
+  while p1 < arr.size || p2 < arr.size
+    if arr[p1] == arr[p2]
+      p2 += 1
+    else
+      size = p2 - p1
+      h[size] += arr[p1...p2].join
+      p1 = p2
+    end
+  end
+
+  result = ''
+  h.keys.sort.reverse.each { |key| result += h[key] }
+
+  result
+end
diff --git a/test/medium/test_451_sort_characters_by_frequency.rb b/test/medium/test_451_sort_characters_by_frequency.rb
new file mode 100644
index 00000000..09e399e3
--- /dev/null
+++ b/test/medium/test_451_sort_characters_by_frequency.rb
@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/medium/451_sort_characters_by_frequency'
+require 'minitest/autorun'
+
+class SortCharactersByFrequencyTest < ::Minitest::Test
+  def test_default_one
+    assert_equal(
+      'eert',
+      frequency_sort451(
+        'tree'
+      )
+    )
+  end
+
+  def test_default_two
+    assert_equal(
+      'aaaccc',
+      frequency_sort451(
+        'cccaaa'
+      )
+    )
+  end
+
+  def test_default_three
+    assert_equal(
+      'bbAa',
+      frequency_sort451(
+        'bbAa'
+      )
+    )
+  end
+end