adding problem set + solution

lib/leetcode.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'leetcode/merge_k_sorted_lists'
+require 'leetcode/reverse_nodes_in_k_group'
 require 'leetcode/median_of_two_sorted_arrays'
 require 'leetcode/regular_expression_matching'
 

lib/leetcode/reverse_nodes_in_k_group.rb

@@ -0,0 +1,25 @@
+module Leetcode
+  # https://leetcode.com/problems/reverse-nodes-in-k-group/
+  class ReverseNodesInKGroup
+    # @param head [Array<Integer>]
+    # @param k [Integer]
+    # @return [Array<Integer>]
+    def execute(head, k)
+      return head if k <= 1
+
+      iterations = (head.length / k)
+      iterations.downto(1) do |iteration|
+        terminus_index = (iteration * k) - 1
+        start_index = terminus_index - k + 1 <= 0 ? 0 : terminus_index - k + 1
+
+        slice = head[start_index..terminus_index].reverse
+
+        terminus_index.downto(start_index) do |swap_index|
+          head[swap_index] = slice.pop
+        end
+      end
+
+      head
+    end
+  end
+end

spec/leetcode/reverse_nodes_in_k_group_spec.rb

@@ -0,0 +1,28 @@
+RSpec.describe Leetcode::ReverseNodesInKGroup do
+  describe '#execute' do
+    let(:head) { [] }
+    let(:k) { [] }
+
+    subject { Leetcode::ReverseNodesInKGroup.new.execute(head, k) }
+
+    context '[1, 2, 3, 4 ,5], 2' do
+      let(:head) { [1, 2, 3, 4 ,5] }
+      let(:k) { 2 }
+
+      it do
+        expect(subject).to be_instance_of(Array)
+        expect(subject).to eql([2, 1, 4, 3, 5])
+      end
+    end
+
+    context '[1, 2, 3, 4, 5], 3' do
+      let(:head) { [1, 2, 3, 4, 5] }
+      let(:k) { 3 }
+
+      it do
+        expect(subject).to be_instance_of(Array)
+        expect(subject).to eql([3, 2, 1, 4, 5])
+      end
+    end
+  end
+end