diff --git a/README.md b/README.md
index 44b8b8e1..60541ad5 100644
--- a/README.md
+++ b/README.md
@@ -531,3 +531,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 179. Largest Number                                             | [Link](https://leetcode.com/problems/largest-number/)                                             | [Link](./lib/medium/179_largest_number.rb)                                             | [Link](./test/medium/test_179_largest_number.rb)                                             |
 | 187. Repeated DNA Sequences                                     | [Link](https://leetcode.com/problems/repeated-dna-sequences/)                                     | [Link](./lib/medium/187_repeated_dna_sequences.rb)                                     | [Link](./test/medium/test_187_repeated_dna_sequences.rb)                                     |
 | 189. Rotate Array                                               | [Link](https://leetcode.com/problems/rotate-array/)                                               | [Link](./lib/medium/189_rotate_array.rb)                                               | [Link](./test/medium/test_189_rotate_array.rb)                                               |
+| 199. Binary Tree Right Side View                                | [Link](https://leetcode.com/problems/binary-tree-right-side-view/)                                | [Link](./lib/medium/199_binary_tree_right_side_view.rb)                                | [Link](./test/medium/test_199_binary_tree_right_side_view.rb)                                |
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index 7dc7fd90..666d482d 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 = '6.6.5'
+  s.version = '6.6.6'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/medium/199_binary_tree_right_side_view.rb b/lib/medium/199_binary_tree_right_side_view.rb
new file mode 100644
index 00000000..f473cf0d
--- /dev/null
+++ b/lib/medium/199_binary_tree_right_side_view.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/binary-tree-right-side-view/
+# @param {TreeNode} root
+# @return {Integer[]}
+def right_side_view(root)
+  return [] unless root
+
+  result = []
+  deque = [root]
+  until deque.empty?
+    size = deque.size
+    until size.zero?
+      node = deque.shift
+      deque << node.left if node.left
+      deque << node.right if node.right
+
+      result << node.val if size == 1
+
+      size -= 1
+    end
+  end
+
+  result
+end
diff --git a/test/medium/test_199_binary_tree_right_side_view.rb b/test/medium/test_199_binary_tree_right_side_view.rb
new file mode 100644
index 00000000..19b358f3
--- /dev/null
+++ b/test/medium/test_199_binary_tree_right_side_view.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/common/binary_tree'
+require_relative '../../lib/medium/199_binary_tree_right_side_view'
+require 'minitest/autorun'
+
+class BinaryTreeRightSideViewTest < ::Minitest::Test
+  def test_default_one
+    assert_equal(
+      [1, 3, 4],
+      right_side_view(
+        ::TreeNode.new(
+          1,
+          ::TreeNode.new(
+            2,
+            nil,
+            ::TreeNode.new(5)
+          ),
+          ::TreeNode.new(
+            3,
+            nil,
+            ::TreeNode.new(4)
+          )
+        )
+      )
+    )
+  end
+
+  def test_default_two
+    assert_equal(
+      [1, 3],
+      right_side_view(
+        ::TreeNode.new(
+          1,
+          nil,
+          ::TreeNode.new(3)
+        )
+      )
+    )
+  end
+
+  def test_default_three
+    assert_equal(
+      [],
+      right_side_view(nil)
+    )
+  end
+end