diff --git a/README.md b/README.md
index 43d0ccef..da41b8b8 100644
--- a/README.md
+++ b/README.md
@@ -649,3 +649,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 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)                      |
+| 239. Sliding Window Maximum                   | [Link](https://leetcode.com/problems/sliding-window-maximum/)                    | [Link](./lib/hard/239_sliding_window_maximum.rb)                   | [Link](./test/hard/test_239_sliding_window_maximum.rb)                   |
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index 2705aab2..3915f243 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 = '7.7.9'
+  s.version = '7.8.0'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/hard/239_sliding_window_maximum.rb b/lib/hard/239_sliding_window_maximum.rb
new file mode 100644
index 00000000..9b6acc7c
--- /dev/null
+++ b/lib/hard/239_sliding_window_maximum.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/sliding-window-maximum/
+# @param {Integer[]} nums
+# @param {Integer} k
+# @return {Integer[]}
+def max_sliding_window(nums, k)
+  result = ::Array.new(nums.size - k + 1)
+  p = 0
+  i = 0
+  j = 0
+  arr = []
+
+  while j < nums.size
+    arr.pop while !arr.empty? && nums[j] > arr.last
+
+    arr << nums[j]
+    size = j - i + 1
+
+    if size >= k
+      result[p] = arr.first
+
+      arr.shift if arr.first == nums[i]
+
+      p += 1
+      i += 1
+    end
+
+    j += 1
+  end
+
+  result
+end
diff --git a/test/hard/test_239_sliding_window_maximum.rb b/test/hard/test_239_sliding_window_maximum.rb
new file mode 100644
index 00000000..d20e0bba
--- /dev/null
+++ b/test/hard/test_239_sliding_window_maximum.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/hard/239_sliding_window_maximum'
+require 'minitest/autorun'
+
+class SlidingWindowMaximumTest < ::Minitest::Test
+  def test_default_one
+    assert_equal(
+      [3, 3, 5, 5, 6, 7],
+      max_sliding_window(
+        [1, 3, -1, -3, 5, 3, 6, 7],
+        3
+      )
+    )
+  end
+
+  def test_default_two
+    assert_equal(
+      [1],
+      max_sliding_window(
+        [1],
+        1
+      )
+    )
+  end
+end