diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index 30a001f9..3e840194 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 = '7.3.9'
+  s.version = '7.3.9.1'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/easy/501_find_mode_in_binary_search_tree.rb b/lib/easy/501_find_mode_in_binary_search_tree.rb
index 6a1721d9..a6fe1e6f 100644
--- a/lib/easy/501_find_mode_in_binary_search_tree.rb
+++ b/lib/easy/501_find_mode_in_binary_search_tree.rb
@@ -4,9 +4,9 @@
 # @param {TreeNode} root
 # @return {Integer[]}
 def find_mode(root)
-  $prev = -1
-  $count = 1
-  $max = -1
+  @prev = -1
+  @count = 1
+  @max = -1
 
   result = []
   _traverse(root, result)
@@ -14,10 +14,6 @@ def find_mode(root)
   result
 end
 
-$prev = -1
-$count = 1
-$max = -1
-
 # @param {TreeNode} node
 # @param {Integer[]} modes
 def _traverse(node, modes)
@@ -25,24 +21,24 @@ def _traverse(node, modes)
 
   _traverse(node.left, modes)
 
-  if $prev != -1
-    if $prev == node.val
-      $count += 1
+  if @prev != -1
+    if @prev == node.val
+      @count += 1
     else
-      $count = 1
+      @count = 1
     end
   end
 
   # noinspection RubyMismatchedArgumentType
-  if $count > $max
-    $max = $count
+  if @count > @max
+    @max = @count
     modes.clear
     modes << node.val
-  elsif $count == $max
+  elsif @count == @max
     modes << node.val
   end
 
-  $prev = node.val
+  @prev = node.val
 
   _traverse(node.right, modes)
 end
diff --git a/lib/easy/543_diameter_of_binary_tree.rb b/lib/easy/543_diameter_of_binary_tree.rb
index b8f8fba3..837b63bf 100644
--- a/lib/easy/543_diameter_of_binary_tree.rb
+++ b/lib/easy/543_diameter_of_binary_tree.rb
@@ -1,15 +1,13 @@
 # frozen_string_literal: true
 
-$max = 0
-
 # https://leetcode.com/problems/diameter-of-binary-tree/
 # @param {TreeNode} root
 # @return {Integer}
 def diameter_of_binary_tree(root)
-  $max = 0
+  @max = 0
   calc_max_depth(root)
 
-  $max
+  @max
 end
 
 # @param {TreeNode} node
@@ -18,7 +16,7 @@ def calc_max_depth(node)
 
   left = calc_max_depth(node.left)
   right = calc_max_depth(node.right)
-  $max = [$max, left + right].max
+  @max = [@max, left + right].max
 
   [left, right].max + 1
 end
diff --git a/lib/easy/563_binary_tree_tilt.rb b/lib/easy/563_binary_tree_tilt.rb
index 1a67923e..d626f2ea 100644
--- a/lib/easy/563_binary_tree_tilt.rb
+++ b/lib/easy/563_binary_tree_tilt.rb
@@ -1,15 +1,13 @@
 # frozen_string_literal: true
 
-$tilt = 0
-
 # https://leetcode.com/problems/binary-tree-tilt/
 # @param {TreeNode} root
 # @return {Integer}
 def find_tilt(root)
-  $tilt = 0
+  @tilt = 0
   calc_tilt(root)
 
-  $tilt
+  @tilt
 end
 
 # @param {TreeNode} node
@@ -20,7 +18,7 @@ def calc_tilt(node)
 
   left = calc_tilt(node.left)
   right = calc_tilt(node.right)
-  $tilt += (left - right).abs
+  @tilt += (left - right).abs
 
   left + right + node.val
 end
diff --git a/lib/easy/671_second_minimum_node_in_a_binary_tree.rb b/lib/easy/671_second_minimum_node_in_a_binary_tree.rb
index 796e3eb4..e70dd082 100644
--- a/lib/easy/671_second_minimum_node_in_a_binary_tree.rb
+++ b/lib/easy/671_second_minimum_node_in_a_binary_tree.rb
@@ -1,27 +1,27 @@
 # frozen_string_literal: true
 
-$r_max = 1 << (1 << 16)
-$f_min = 0
-$s_min = $max
-
 # https://leetcode.com/problems/second-minimum-node-in-a-binary-tree/
 # @param {TreeNode} root
 # @return {Integer}
 def find_second_minimum_value(root)
-  $f_min = root.val
-  $s_min = $r_max
+  @r_max = 1 << (1 << 16)
+  @f_min = 0
+  @s_min = @max
+
+  @f_min = root.val
+  @s_min = @r_max
   find_second_minimum(root)
 
-  $s_min < $r_max ? $s_min : -1
+  @s_min < @r_max ? @s_min : -1
 end
 
 # @param {TreNode} node
 def find_second_minimum(node)
   return if node.nil?
 
-  if $f_min < node.val && node.val < $s_min
-    $s_min = node.val
-  elsif $f_min == node.val
+  if @f_min < node.val && node.val < @s_min
+    @s_min = node.val
+  elsif @f_min == node.val
     find_second_minimum(node.left)
     find_second_minimum(node.right)
   end
diff --git a/lib/easy/897_increasing_order_search_tree.rb b/lib/easy/897_increasing_order_search_tree.rb
index ff42d470..0f95f5ea 100644
--- a/lib/easy/897_increasing_order_search_tree.rb
+++ b/lib/easy/897_increasing_order_search_tree.rb
@@ -7,13 +7,13 @@
 # @return {TreeNode}
 def increasing_bst(root)
   result = ::TreeNode.new
-  $curr = result
+  @curr = result
   inorder_iost(root)
 
   result.right
 end
 
-$curr = nil
+private
 
 # @param {TreeNode} node
 def inorder_iost(node)
@@ -21,7 +21,7 @@ def inorder_iost(node)
 
   inorder_iost(node.left)
   node.left = nil
-  $curr.right = node
-  $curr = node
+  @curr.right = node
+  @curr = node
   inorder_iost(node.right)
 end
diff --git a/lib/medium/105_construct_binary_tree_from_preorder_and_inorder_traversal.rb b/lib/medium/105_construct_binary_tree_from_preorder_and_inorder_traversal.rb
index e54d8edc..168cc91e 100644
--- a/lib/medium/105_construct_binary_tree_from_preorder_and_inorder_traversal.rb
+++ b/lib/medium/105_construct_binary_tree_from_preorder_and_inorder_traversal.rb
@@ -2,18 +2,14 @@
 
 require_relative '../common/binary_tree'
 
-$pre_index = 0
-
 # https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
 # @param {Integer[]} preorder
 # @param {Integer[]} inorder
 # @return {TreeNode}
 def build_tree(preorder, inorder)
-  $pre_index = 0
+  @pre_index = 0
   in_index = {}
-  (0...inorder.size).each do |i|
-    in_index[inorder[i]] = i
-  end
+  (0...inorder.size).each { |i| in_index[inorder[i]] = i }
 
   array_to_tree(preorder, 0, preorder.size - 1, in_index)
 end
@@ -27,8 +23,8 @@ def build_tree(preorder, inorder)
 def array_to_tree(pre, l, r, in_index)
   return if l > r
 
-  val = pre[$pre_index]
-  $pre_index += 1
+  val = pre[@pre_index]
+  @pre_index += 1
 
   root = ::TreeNode.new(val)
   root.left = array_to_tree(pre, l, in_index[val] - 1, in_index)
diff --git a/lib/medium/508_most_frequent_subtree_sum.rb b/lib/medium/508_most_frequent_subtree_sum.rb
index 314d79dd..7d43ab94 100644
--- a/lib/medium/508_most_frequent_subtree_sum.rb
+++ b/lib/medium/508_most_frequent_subtree_sum.rb
@@ -1,18 +1,16 @@
 # frozen_string_literal: true
 
-$freq = 0
-
 # https://leetcode.com/problems/most-frequent-subtree-sum/
 # @param {TreeNode} root
 # @return {Integer[]}
 def find_frequent_tree_sum(root)
-  $freq = 0
+  @freq = 0
 
   values = {}
   fill_values_from_sub_tree(root, values)
 
   most_freq = []
-  values.entries.each { |key, value| most_freq << key if value == $freq }
+  values.entries.each { |key, value| most_freq << key if value == @freq }
 
   most_freq
 end
@@ -30,7 +28,7 @@ def fill_values_from_sub_tree(node, values)
   sum = node.val + left + right
 
   values[sum] = values.fetch(sum, 0) + 1
-  $freq = [$freq, values[sum]].max
+  @freq = [@freq, values[sum]].max
 
   sum
 end
diff --git a/lib/medium/513_find_bottom_left_tree_value.rb b/lib/medium/513_find_bottom_left_tree_value.rb
index e52216cb..f7e4476b 100644
--- a/lib/medium/513_find_bottom_left_tree_value.rb
+++ b/lib/medium/513_find_bottom_left_tree_value.rb
@@ -1,18 +1,15 @@
 # frozen_string_literal: true
 
-$path = -1
-$value = 0
-
 # https://leetcode.com/problems/find-bottom-left-tree-value/
 # @param {TreeNode} root
 # @return {Integer}
 def find_bottom_left_value(root)
-  $path = -1
-  $value = 0
+  @path = -1
+  @value = 0
 
   apply_to_find_bottom_left_value(root, 0)
 
-  $value
+  @value
 end
 
 private
@@ -23,9 +20,9 @@ def find_bottom_left_value(root)
 def apply_to_find_bottom_left_value(node, curr)
   return unless node
 
-  if $path < curr
-    $path = curr
-    $value = node.val
+  if @path < curr
+    @path = curr
+    @value = node.val
   end
 
   apply_to_find_bottom_left_value(node.left, curr + 1)
diff --git a/lib/medium/535_encode_and_decode_tinyurl.rb b/lib/medium/535_encode_and_decode_tinyurl.rb
index a6286985..69f818c4 100644
--- a/lib/medium/535_encode_and_decode_tinyurl.rb
+++ b/lib/medium/535_encode_and_decode_tinyurl.rb
@@ -1,15 +1,15 @@
 # frozen_string_literal: true
 
-$urls = {}
-
 # https://leetcode.com/problems/encode-and-decode-tinyurl/
 # @param {String} long_url
 # @return {String}
 def encode(long_url)
+  @urls = {}
+
   short_url = generate_short_url(long_url)
-  short_url = generate_short_url(long_url) while $urls.include?(short_url)
+  short_url = generate_short_url(long_url) while @urls.include?(short_url)
 
-  $urls[short_url] = long_url
+  @urls[short_url] = long_url
 
   short_url
 end
@@ -27,4 +27,4 @@ def generate_short_url(_long_url)
 
 # @param {String} short_url
 # @return {String}
-def decode(short_url) = $urls[short_url]
+def decode(short_url) = @urls[short_url]
diff --git a/lib/medium/538_convert_bst_to_greater_tree.rb b/lib/medium/538_convert_bst_to_greater_tree.rb
index 24c75753..63f0ed63 100644
--- a/lib/medium/538_convert_bst_to_greater_tree.rb
+++ b/lib/medium/538_convert_bst_to_greater_tree.rb
@@ -1,12 +1,10 @@
 # frozen_string_literal: true
 
-$sum = 0
-
 # https://leetcode.com/problems/convert-bst-to-greater-tree/
 # @param {TreeNode} root
 # @return {TreeNode}
 def convert_bst(root)
-  $sum = 0
+  @sum = 0
 
   convert_bst_with_sum(root)
 end
@@ -19,8 +17,8 @@ def convert_bst_with_sum(node)
   return unless node
 
   convert_bst_with_sum(node.right)
-  $sum += node.val
-  node.val = $sum
+  @sum += node.val
+  node.val = @sum
   convert_bst_with_sum(node.left)
 
   node
diff --git a/test/easy/test_278_first_bad_version.rb b/test/easy/test_278_first_bad_version.rb
index ba554c8f..a315b87a 100644
--- a/test/easy/test_278_first_bad_version.rb
+++ b/test/easy/test_278_first_bad_version.rb
@@ -7,11 +7,13 @@
 class FirstBadVersionTest < ::Minitest::Test
   def test_default_one
     $b_version = 4
+
     assert_equal(4, first_bad_version(5))
   end
 
   def test_default_two
     $b_version = 1
+
     assert_equal(1, first_bad_version(1))
   end
 end
diff --git a/test/easy/test_374_guess_number_higher_or_lower.rb b/test/easy/test_374_guess_number_higher_or_lower.rb
index c5069374..b328d39b 100644
--- a/test/easy/test_374_guess_number_higher_or_lower.rb
+++ b/test/easy/test_374_guess_number_higher_or_lower.rb
@@ -7,16 +7,19 @@
 class GuessNumberHigherOrLowerTest < ::Minitest::Test
   def test_default_one
     $guess_num = 6
+
     assert_equal(6, guess_number(10))
   end
 
   def test_default_two
     $guess_num = 1
+
     assert_equal(1, guess_number(1))
   end
 
   def test_default_three
     $guess_num = 1
+
     assert_equal(1, guess_number(2))
   end
 end