diff --git a/README.md b/README.md
index 772eea59..6a80e56d 100644
--- a/README.md
+++ b/README.md
@@ -563,3 +563,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 382. Linked List Random Node                                    | [Link](https://leetcode.com/problems/linked-list-random-node/)                                    | [Link](./lib/medium/382_linked_list_random_node.rb)                                    | [Link](./test/medium/test_382_linked_list_random_node.rb)                                    |
 | 388. Longest Absolute File Path                                 | [Link](https://leetcode.com/problems/longest-absolute-file-path/)                                 | [Link](./lib/medium/388_longest_absolute_file_path.rb)                                 | [Link](./test/medium/test_388_longest_absolute_file_path.rb)                                 |
 | 394. Decode String                                              | [Link](https://leetcode.com/problems/decode-string/)                                              | [Link](./lib/medium/394_decode_string.rb)                                              | [Link](./test/medium/test_394_decode_string.rb)                                              |
+| 398. Random Pick Index                                          | [Link](https://leetcode.com/problems/random-pick-index/)                                          | [Link](./lib/medium/398_random_pick_index.rb)                                          | [Link](./test/medium/test_398_random_pick_index.rb)                                          |
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index bcf2320e..7278ea0d 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 = '6.9.7'
+  s.version = '6.9.8'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/medium/398_random_pick_index.rb b/lib/medium/398_random_pick_index.rb
new file mode 100644
index 00000000..e8f5b4d8
--- /dev/null
+++ b/lib/medium/398_random_pick_index.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/random-pick-index/
+class RandomPickIndex
+  # @param {Integer[]} nums
+  def initialize(nums)
+    @nums = nums
+    @indices = ::Hash.new { |hash, key| hash[key] = [] }
+  end
+
+  # @param {Integer} target
+  # @return {Integer}
+  def pick(target)
+    if @indices.empty?
+      @nums.each_with_index do |num, index|
+        @indices[num] << index
+      end
+    end
+
+    picks = @indices[target]
+
+    picks[rand(picks.size)]
+  end
+end
diff --git a/test/medium/test_398_random_pick_index.rb b/test/medium/test_398_random_pick_index.rb
new file mode 100644
index 00000000..427aeb3b
--- /dev/null
+++ b/test/medium/test_398_random_pick_index.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/medium/398_random_pick_index'
+require 'minitest/autorun'
+
+class RandomPickIndexTest < ::Minitest::Test
+  def test_default_one
+    random_pick_index = ::RandomPickIndex.new([1, 2, 3, 3, 3])
+    assert([2, 3, 4].include?(random_pick_index.pick(3)))
+    assert([0].include?(random_pick_index.pick(1)))
+    assert([2, 3, 4].include?(random_pick_index.pick(3)))
+  end
+end