Merge pull request #716 from fartem/142_Linked_List_Cycle_II

2024-08-21 v. 6.5.2: added "142. Linked List Cycle II"

Author: fartem

README.md

@@ -517,3 +517,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 133. Clone Graph                                                | [Link](https://leetcode.com/problems/clone-graph/)                                                | [Link](./lib/medium/133_clone_graph.rb)                                                |
 | 134. Gas Station                                                | [Link](https://leetcode.com/problems/gas-station/)                                                | [Link](./lib/medium/134_gas_station.rb)                                                |
 | 138. Copy List with Random Pointer                              | [Link](https://leetcode.com/problems/copy-list-with-random-pointer/)                              | [Link](./lib/medium/138_copy_list_with_random_pointer.rb)                              |
+| 142. Linked List Cycle II                                       | [Link](https://leetcode.com/problems/linked-list-cycle-ii/)                                       | [Link](./lib/medium/142_linked_list_cycle_ii.rb)                                       |

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 = '6.5.1.2'
+  s.version = '6.5.2'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'

lib/medium/142_linked_list_cycle_ii.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/linked-list-cycle-ii/
+# @param {ListNode} head
+# @return {ListNode}
+def detect_cycle(head)
+  tort = head
+  hare = head
+  is_cycle = false
+  while tort && hare && hare.next
+    tort = tort.next
+    hare = hare.next.next
+
+    next unless hare == tort
+
+    is_cycle = true
+
+    break
+  end
+
+  return unless is_cycle
+
+  tort = head
+  until tort == hare
+    tort = tort.next
+    hare = hare.next
+  end
+
+  tort
+end

test/medium/test_142_linked_list_cycle_ii.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/common/linked_list'
+require_relative '../../lib/medium/142_linked_list_cycle_ii'
+require 'minitest/autorun'
+
+class LinkedListCycleIITest < ::Minitest::Test
+  def test_default_one
+    start = ::ListNode.new(2)
+    middle = ::ListNode.new(0)
+    start.next = middle
+    middle.next = ::ListNode.new(-4, start)
+    head = ::ListNode.new(3, start)
+
+    assert_equal(
+      start,
+      detect_cycle(
+        head
+      )
+    )
+  end
+
+  def test_default_two
+    start = ::ListNode.new(1)
+    nd = ::ListNode.new(2)
+    start.next = nd
+    nd.next = start
+
+    assert_equal(
+      start,
+      detect_cycle(
+        start
+      )
+    )
+  end
+
+  def test_default_three
+    assert_nil(
+      detect_cycle(
+        ::ListNode.new(1)
+      )
+    )
+  end
+end