diff --git a/README.md b/README.md
index 639fd189..ec96a47e 100644
--- a/README.md
+++ b/README.md
@@ -646,3 +646,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 41. First Missing Positive                    | [Link](https://leetcode.com/problems/first-missing-positive/)                    | [Link](./lib/hard/41_first_missing_positive.rb)                    | [Link](./test/hard/test_41_first_missing_positive.rb)                    |
 | 115. Distinct Subsequences                    | [Link](https://leetcode.com/problems/distinct-subsequences/)                     | [Link](./lib/hard/115_distinct_subsequences.rb)                    | [Link](./test/hard/test_115_distinct_subsequences.rb)                    |
 | 126. Word Ladder II                           | [Link](https://leetcode.com/problems/word-ladder-ii/)                            | [Link](./lib/hard/126_word_ladder_ii.rb)                           | [Link](./test/hard/test_126_word_ladder_ii.rb)                           |
+| 233. Number of Digit One                      | [Link](https://leetcode.com/problems/number-of-digit-one/)                       | [Link](./lib/hard/233_number_of_digit_one.rb)                      | [Link](./test/hard/test_233_number_of_digit_one.rb)                      |
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index a1565372..4c3b6c75 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 = '7.7.6'
+  s.version = '7.7.7'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/hard/233_number_of_digit_one.rb b/lib/hard/233_number_of_digit_one.rb
new file mode 100644
index 00000000..42f8a5aa
--- /dev/null
+++ b/lib/hard/233_number_of_digit_one.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/number-of-digit-one/
+# @param {Integer} n
+# @return {Integer}
+def count_digit_one(n)
+  result = 0
+  i = 1
+  while i <= n
+    div = i * 10
+    result += (n / div) * i + [[n % div - i + 1, 0].max, i].min
+
+    i *= 10
+  end
+
+  result
+end
diff --git a/test/hard/test_233_number_of_digit_one.rb b/test/hard/test_233_number_of_digit_one.rb
new file mode 100644
index 00000000..5cc0758b
--- /dev/null
+++ b/test/hard/test_233_number_of_digit_one.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/hard/233_number_of_digit_one'
+require 'minitest/autorun'
+
+class NumberOfDigitOneTest < ::Minitest::Test
+  def test_default_one = assert_equal(6, count_digit_one(13))
+
+  def test_default_two = assert_equal(0, count_digit_one(0))
+end