diff --git a/README.md b/README.md
index e2fcf25a..ec78da99 100644
--- a/README.md
+++ b/README.md
@@ -538,3 +538,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 209. Minimum Size Subarray Sum                                  | [Link](https://leetcode.com/problems/minimum-size-subarray-sum/)                                  | [Link](./lib/medium/209_minimum_size_subarray_sum.rb)                                  | [Link](./test/medium/test_209_minimum_size_subarray_sum.rb)                                  |
 | 215. Kth Largest Element in an Array                            | [Link](https://leetcode.com/problems/kth-largest-element-in-an-array/)                            | [Link](./lib/medium/215_kth_largest_element_in_an_array.rb)                            | [Link](./test/medium/test_215_kth_largest_element_in_an_array.rb)                            |
 | 227. Basic Calculator II                                        | [Link](https://leetcode.com/problems/basic-calculator-ii/)                                        | [Link](./lib/medium/227_basic_calculator_ii.rb)                                        | [Link](./test/medium/test_227_basic_calculator_ii.rb)                                        |
+| 230. Kth Smallest Element in a BST                              | [Link](https://leetcode.com/problems/kth-smallest-element-in-a-bst/)                              | [Link](./lib/medium/230_kth_smallest_element_in_a_bst.rb)                              | [Link](./test/medium/test_230_kth_smallest_element_in_a_bst.rb)                              |
diff --git a/Rakefile b/Rakefile
index 4bc26677..3dfd8d6e 100644
--- a/Rakefile
+++ b/Rakefile
@@ -2,7 +2,7 @@
 
 require 'rake/clean'
 
-task default: %i[clean rubocop test]
+task default: %i[clean rubocop test ci]
 
 desc 'Run RuboCop'
 require 'rubocop/rake_task'
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index efd990a1..e83e64d6 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 = '6.7.2'
+  s.version = '6.7.3'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/medium/230_kth_smallest_element_in_a_bst.rb b/lib/medium/230_kth_smallest_element_in_a_bst.rb
new file mode 100644
index 00000000..f6556ee4
--- /dev/null
+++ b/lib/medium/230_kth_smallest_element_in_a_bst.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/kth-smallest-element-in-a-bst/
+# @param {TreeNode} root
+# @param {Integer} k
+# @return {Integer}
+def kth_smallest(root, k)
+  nums = []
+  apply230(root, nums)
+
+  nums[k - 1]
+end
+
+private
+
+# @param {TreeNode} root
+# @param {Array} nums
+# @return {Void}
+def apply230(node, nums)
+  return unless node
+
+  apply230(node.left, nums)
+  nums << node.val
+  apply230(node.right, nums)
+end
diff --git a/test/medium/test_230_kth_smallest_element_in_a_bst.rb b/test/medium/test_230_kth_smallest_element_in_a_bst.rb
new file mode 100644
index 00000000..d6b25e1b
--- /dev/null
+++ b/test/medium/test_230_kth_smallest_element_in_a_bst.rb
@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/common/binary_tree'
+require_relative '../../lib/medium/230_kth_smallest_element_in_a_bst'
+require 'minitest/autorun'
+
+class KthSmallestElementInABSTTest < ::Minitest::Test
+  def test_default_one
+    assert_equal(
+      1,
+      kth_smallest(
+        ::TreeNode.new(
+          3,
+          ::TreeNode.new(
+            1,
+            nil,
+            ::TreeNode.new(2)
+          ),
+          ::TreeNode.new(4)
+        ),
+        1
+      )
+    )
+  end
+
+  def test_default_two
+    assert_equal(
+      3,
+      kth_smallest(
+        ::TreeNode.new(
+          5,
+          ::TreeNode.new(
+            3,
+            ::TreeNode.new(
+              2,
+              ::TreeNode.new(1),
+              nil
+            ),
+            ::TreeNode.new(4)
+          ),
+          ::TreeNode.new(6)
+        ),
+        3
+      )
+    )
+  end
+end