Linked List Cycle II
<p>Given the <code>head</code> of a linked list, return <em>the node where the cycle begins. If there is no cycle, return </em><code>null</code>.</p>

<p>There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the <code>next</code> pointer. Internally, <code>pos</code> is used to denote the index of the node that tail&#39;s <code>next</code> pointer is connected to (<strong>0-indexed</strong>). It is <code>-1</code> if there is no cycle. <strong>Note that</strong> <code>pos</code> <strong>is not passed as a parameter</strong>.</p>

<p><strong>Do not modify</strong> the linked list.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png" style="height: 145px; width: 450px;" />
<pre>
<strong>Input:</strong> head = [3,2,0,-4], pos = 1
<strong>Output:</strong> tail connects to node index 1
<strong>Explanation:</strong> There is a cycle in the linked list, where tail connects to the second node.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png" style="height: 105px; width: 201px;" />
<pre>
<strong>Input:</strong> head = [1,2], pos = 0
<strong>Output:</strong> tail connects to node index 0
<strong>Explanation:</strong> There is a cycle in the linked list, where tail connects to the first node.
</pre>

<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png" style="height: 65px; width: 65px;" />
<pre>
<strong>Input:</strong> head = [1], pos = -1
<strong>Output:</strong> no cycle
<strong>Explanation:</strong> There is no cycle in the linked list.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of the nodes in the list is in the range <code>[0, 10<sup>4</sup>]</code>.</li>
	<li><code>-10<sup>5</sup> &lt;= Node.val &lt;= 10<sup>5</sup></code></li>
	<li><code>pos</code> is <code>-1</code> or a <strong>valid index</strong> in the linked-list.</li>
</ul>

<p>&nbsp;</p>
<p><strong>Follow up:</strong> Can you solve it using <code>O(1)</code> (i.e. constant) memory?</p>

['linked-list-cycle', 'find-the-duplicate-number']
[]
[{"value": "cpp", "text": "C++", "defaultCode": "/**\n * Definition for singly-linked list.\n * struct ListNode {\n *     int val;\n *     ListNode *next;\n *     ListNode(int x) : val(x), next(NULL) {}\n * };\n */\nclass Solution {\npublic:\n    ListNode *detectCycle(ListNode *head) {\n        \n    }\n};"}, {"value": "java", "text": "Java", "defaultCode": "/**\n * Definition for singly-linked list.\n * class ListNode {\n *     int val;\n *     ListNode next;\n *     ListNode(int x) {\n *         val = x;\n *         next = null;\n *     }\n * }\n */\npublic class Solution {\n    public ListNode detectCycle(ListNode head) {\n        \n    }\n}"}, {"value": "python", "text": "Python", "defaultCode": "# Definition for singly-linked list.\n# class ListNode(object):\n#     def __init__(self, x):\n#         self.val = x\n#         self.next = None\n\nclass Solution(object):\n    def detectCycle(self, head):\n        \"\"\"\n        :type head: ListNode\n        :rtype: ListNode\n        \"\"\"\n        "}, {"value": "python3", "text": "Python3", "defaultCode": "# Definition for singly-linked list.\n# class ListNode:\n#     def __init__(self, x):\n#         self.val = x\n#         self.next = None\n\nclass Solution:\n    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:\n        "}, {"value": "c", "text": "C", "defaultCode": "/**\n * Definition for singly-linked list.\n * struct ListNode {\n *     int val;\n *     struct ListNode *next;\n * };\n */\nstruct ListNode *detectCycle(struct ListNode *head) {\n    \n}"}, {"value": "csharp", "text": "C#", "defaultCode": "/**\n * Definition for singly-linked list.\n * public class ListNode {\n *     public int val;\n *     public ListNode next;\n *     public ListNode(int x) {\n *         val = x;\n *         next = null;\n *     }\n * }\n */\npublic class Solution {\n    public ListNode DetectCycle(ListNode head) {\n        \n    }\n}"}, {"value": "javascript", "text": "JavaScript", "defaultCode": "/**\n * Definition for singly-linked list.\n * function ListNode(val) {\n *     this.val = val;\n *     this.next = null;\n * }\n */\n\n/**\n * @param {ListNode} head\n * @return {ListNode}\n */\nvar detectCycle = function(head) {\n    \n};"}, {"value": "typescript", "text": "TypeScript", "defaultCode": "/**\n * Definition for singly-linked list.\n * class ListNode {\n *     val: number\n *     next: ListNode | null\n *     constructor(val?: number, next?: ListNode | null) {\n *         this.val = (val===undefined ? 0 : val)\n *         this.next = (next===undefined ? null : next)\n *     }\n * }\n */\n\nfunction detectCycle(head: ListNode | null): ListNode | null {\n    \n};"}, {"value": "php", "text": "PHP", "defaultCode": "/**\n * Definition for a singly-linked list.\n * class ListNode {\n *     public $val = 0;\n *     public $next = null;\n *     function __construct($val) { $this->val = $val; }\n * }\n */\n\nclass Solution {\n    /**\n     * @param ListNode $head\n     * @return ListNode\n     */\n    function detectCycle($head) {\n        \n    }\n}"}, {"value": "swift", "text": "Swift", "defaultCode": "/**\n * Definition for singly-linked list.\n * public class ListNode {\n *     public var val: Int\n *     public var next: ListNode?\n *     public init(_ val: Int) {\n *         self.val = val\n *         self.next = nil\n *     }\n * }\n */\n\nclass Solution {\n    func detectCycle(_ head: ListNode?) -> ListNode? {\n        \n    }\n}"}, {"value": "kotlin", "text": "Kotlin", "defaultCode": "/**\n * Example:\n * var li = ListNode(5)\n * var v = li.`val`\n * Definition for singly-linked list.\n * class ListNode(var `val`: Int) {\n *     var next: ListNode? = null\n * }\n */\n\nclass Solution {\n    fun detectCycle(head: ListNode?): ListNode? {\n        \n    }\n}"}, {"value": "golang", "text": "Go", "defaultCode": "/**\n * Definition for singly-linked list.\n * type ListNode struct {\n *     Val int\n *     Next *ListNode\n * }\n */\nfunc detectCycle(head *ListNode) *ListNode {\n    \n}"}, {"value": "ruby", "text": "Ruby", "defaultCode": "# Definition for singly-linked list.\n# class ListNode\n#     attr_accessor :val, :next\n#     def initialize(val)\n#         @val = val\n#         @next = nil\n#     end\n# end\n\n# @param {ListNode} head\n# @return {ListNode}\ndef detectCycle(head)\n    \nend"}, {"value": "scala", "text": "Scala", "defaultCode": "/**\n * Definition for singly-linked list.\n * class ListNode(var _x: Int = 0) {\n *   var next: ListNode = null\n *   var x: Int = _x\n * }\n */\n\nobject Solution {\n    def detectCycle(head: ListNode): ListNode = {\n        \n    }\n}"}]
{
  "name": "detectCycle",
  "params": [
    {
      "name": "head",
      "type": "ListNode"
    },
    {
      "name": "pos",
      "type": "integer"
    }
  ],
  "return": {
    "type": "ListNode"
  },
  "languages": [
    "cpp",
    "java",
    "python",
    "c",
    "csharp",
    "javascript",
    "golang",
    "python3",
    "swift",
    "kotlin",
    "php",
    "ruby",
    "scala",
    "typescript"
  ],
  "manual": true
}
{"cpp": ["C++", "<p>Compiled with <code> clang 19 </code> using the latest C++ 23 standard, and <code>libstdc++</code> provided by GCC 14.</p>\r\n\r\n<p>Your code is compiled with level two optimization (<code>-O2</code>). <a href=\"https://github.com/google/sanitizers/wiki/AddressSanitizer\" target=\"_blank\">AddressSanitizer</a> is also enabled to help detect out-of-bounds and use-after-free bugs.</p>\r\n\r\n<p>Most standard library headers are already included automatically for your convenience.</p>"], "java": ["Java", "<p><code>OpenJDK 21</code>. Using compile arguments: <code>--enable-preview --release 21</code></p>\r\n\r\n<p>Most standard library headers are already included automatically for your convenience.</p>\r\n<p>Includes <code>Pair</code> class from https://docs.oracle.com/javase/8/javafx/api/javafx/util/Pair.html.</p>"], "python": ["Python", "<p><code>Python 2.7.18</code>.</p>\r\n\r\n<p>Most libraries are already imported automatically for your convenience, such as <a href=\"https://docs.python.org/2/library/array.html\" target=\"_blank\">array</a>, <a href=\"https://docs.python.org/2/library/bisect.html\" target=\"_blank\">bisect</a>, <a href=\"https://docs.python.org/2/library/collections.html\" target=\"_blank\">collections</a>. If you need more libraries, you can import it yourself.</p>\r\n\r\n<p>For Map/TreeMap data structure, you may use <a href=\"http://www.grantjenks.com/docs/sortedcontainers/\" target=\"_blank\">sortedcontainers</a> library.</p>\r\n\r\n<p>Note that Python 2.7 <a href=\"https://www.python.org/dev/peps/pep-0373/\" target=\"_blank\">is not maintained anymore</a>. For the latest Python, please choose Python3 instead.</p>"], "c": ["C", "<p>Compiled with <code>gcc 14</code> using the gnu11 standard.</p>\r\n\r\n<p>Your code is compiled with level one optimization (<code>-O2</code>). <a href=\"https://github.com/google/sanitizers/wiki/AddressSanitizer\" target=\"_blank\">AddressSanitizer</a> is also enabled to help detect out-of-bounds and use-after-free bugs.</p>\r\n\r\n<p>Most standard library headers are already included automatically for your convenience.</p>\r\n\r\n<p>For hash table operations, you may use <a href=\"https://troydhanson.github.io/uthash/\" target=\"_blank\">uthash</a>. \"uthash.h\" is included by default. Below are some examples:</p>\r\n\r\n<p><b>1. Adding an item to a hash.</b>\r\n<pre>\r\nstruct hash_entry {\r\n    int id;            /* we'll use this field as the key */\r\n    char name[10];\r\n    UT_hash_handle hh; /* makes this structure hashable */\r\n};\r\n\r\nstruct hash_entry *users = NULL;\r\n\r\nvoid add_user(struct hash_entry *s) {\r\n    HASH_ADD_INT(users, id, s);\r\n}\r\n</pre>\r\n</p>\r\n\r\n<p><b>2. Looking up an item in a hash:</b>\r\n<pre>\r\nstruct hash_entry *find_user(int user_id) {\r\n    struct hash_entry *s;\r\n    HASH_FIND_INT(users, &user_id, s);\r\n    return s;\r\n}\r\n</pre>\r\n</p>\r\n\r\n<p><b>3. Deleting an item in a hash:</b>\r\n<pre>\r\nvoid delete_user(struct hash_entry *user) {\r\n    HASH_DEL(users, user);  \r\n}\r\n</pre>\r\n</p>"], "csharp": ["C#", "<p><a href=\"https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-13\" target=\"_blank\">C# 13</a> with .NET 9 runtime</p>"], "javascript": ["JavaScript", "<p><code>Node.js 22.14.0</code>.</p>\r\n\r\n<p>Your code is run with <code>--harmony</code> flag, enabling <a href=\"http://node.green/\" target=\"_blank\">new ES6 features</a>.</p>\r\n\r\n<p><a href=\"https://lodash.com\" target=\"_blank\">lodash.js</a> library is included by default.</p>\r\n\r\n<p>For Priority Queue / Queue data structures, you may use 6.3.2 version of <a href=\"https://github.com/datastructures-js/priority-queue/tree/v6.3.2\" target=\"_blank\">datastructures-js/priority-queue</a> and 4.2.3 version of <a href=\"https://github.com/datastructures-js/queue/tree/v4.2.3\" target=\"_blank\">datastructures-js/queue</a> and 1.0.4 version of <a href=\"https://github.com/datastructures-js/deque/tree/v1.0.4\" target=\"_blank\">datastructures-js/deque</a>.</p>"], "ruby": ["Ruby", "<p><code>Ruby 3.2</code></p>\r\n\r\n<p>Some common data structure implementations are provided in the Algorithms module: https://www.rubydoc.info/github/kanwei/algorithms/Algorithms</p>"], "swift": ["Swift", "<p><code>Swift 6.0</code>.</p>\r\n\r\n<p>You may use the following packages:<br/>\r\n<a href=\"https://github.com/apple/swift-algorithms/tree/1.2.0\" target=\"_blank\">swift-algorithms 1.2.0</a><br/>\r\n<a href=\"https://github.com/apple/swift-collections/tree/1.1.4\" target=\"_blank\">swift-collections 1.1.4</a><br/>\r\n<a href=\"https://github.com/apple/swift-numerics/tree/1.0.2\" target=\"_blank\">swift-numerics 1.0.2</a></p>"], "golang": ["Go", "<p><code>Go 1.23</code></p>\r\n<p>Support <a href=\"https://pkg.go.dev/github.com/emirpasic/gods@v1.18.1\" target=\"_blank\">https://pkg.go.dev/github.com/emirpasic/gods@v1.18.1</a> and <a href=\"https://pkg.go.dev/github.com/emirpasic/gods/v2@v2.0.0-alpha\" target=\"_blank\">https://pkg.go.dev/github.com/emirpasic/gods/v2@v2.0.0-alpha</a> library.</p>"], "python3": ["Python3", "<p><code>Python 3.11</code>.</p>\r\n\r\n<p>Most libraries are already imported automatically for your convenience, such as <a href=\"https://docs.python.org/3/library/array.html\" target=\"_blank\">array</a>, <a href=\"https://docs.python.org/3/library/bisect.html\" target=\"_blank\">bisect</a>, <a href=\"https://docs.python.org/3/library/collections.html\" target=\"_blank\">collections</a>. If you need more libraries, you can import it yourself.</p>\r\n\r\n<p>For Map/TreeMap data structure, you may use <a href=\"http://www.grantjenks.com/docs/sortedcontainers/\" target=\"_blank\">sortedcontainers</a> library.</p>"], "scala": ["Scala", "<p><code>Scala 3.3.1</code>.</p>"], "kotlin": ["Kotlin", "<p><code>Kotlin 2.1.10</code>.</p>"], "php": ["PHP", "<p><code>PHP 8.2</code>.</p>\r\n<p>With bcmath module</p>"], "typescript": ["Typescript", "<p><code>TypeScript 5.7.3, Node.js 22.14.0</code>.</p>\r\n\r\n<p>Compile Options: <code>--alwaysStrict --strictBindCallApply --strictFunctionTypes --target ES2024</code></p>\r\n\r\n<p><a href=\"https://lodash.com\" target=\"_blank\">lodash.js</a> library is included by default.</p>\r\n\r\n<p>For Priority Queue / Queue data structures, you may use 6.3.2 version of <a href=\"https://github.com/datastructures-js/priority-queue/tree/v6.3.2\" target=\"_blank\">datastructures-js/priority-queue</a> and 4.2.3 version of <a href=\"https://github.com/datastructures-js/queue/tree/v4.2.3\" target=\"_blank\">datastructures-js/queue</a> and 1.0.4 version of <a href=\"https://github.com/datastructures-js/deque/tree/v1.0.4\" target=\"_blank\">datastructures-js/deque</a>.</p>"]}
['Hash Table', 'Linked List', 'Two Pointers']