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        "question": {
            "questionId": "2196",
            "questionFrontendId": "2074",
            "boundTopicId": null,
            "title": "Reverse Nodes in Even Length Groups",
            "titleSlug": "reverse-nodes-in-even-length-groups",
            "content": "<p>You are given the <code>head</code> of a linked list.</p>\n\n<p>The nodes in the linked list are <strong>sequentially</strong> assigned to <strong>non-empty</strong> groups whose lengths form the sequence of the natural numbers (<code>1, 2, 3, 4, ...</code>). The <strong>length</strong> of a group is the number of nodes assigned to it. In other words,</p>\n\n<ul>\n\t<li>The <code>1<sup>st</sup></code> node is assigned to the first group.</li>\n\t<li>The <code>2<sup>nd</sup></code> and the <code>3<sup>rd</sup></code> nodes are assigned to the second group.</li>\n\t<li>The <code>4<sup>th</sup></code>, <code>5<sup>th</sup></code>, and <code>6<sup>th</sup></code> nodes are assigned to the third group, and so on.</li>\n</ul>\n\n<p>Note that the length of the last group may be less than or equal to <code>1 + the length of the second to last group</code>.</p>\n\n<p><strong>Reverse</strong> the nodes in each group with an <strong>even</strong> length, and return <em>the</em> <code>head</code> <em>of the modified linked list</em>.</p>\n\n<p>&nbsp;</p>\n<p><strong>Example 1:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2021/10/25/eg1.png\" style=\"width: 699px; height: 124px;\" />\n<pre>\n<strong>Input:</strong> head = [5,2,6,3,9,1,7,3,8,4]\n<strong>Output:</strong> [5,6,2,3,9,1,4,8,3,7]\n<strong>Explanation:</strong>\n- The length of the first group is 1, which is odd, hence no reversal occurs.\n- The length of the second group is 2, which is even, hence the nodes are reversed.\n- The length of the third group is 3, which is odd, hence no reversal occurs.\n- The length of the last group is 4, which is even, hence the nodes are reversed.\n</pre>\n\n<p><strong>Example 2:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2021/10/25/eg2.png\" style=\"width: 284px; height: 114px;\" />\n<pre>\n<strong>Input:</strong> head = [1,1,0,6]\n<strong>Output:</strong> [1,0,1,6]\n<strong>Explanation:</strong>\n- The length of the first group is 1. No reversal occurs.\n- The length of the second group is 2. The nodes are reversed.\n- The length of the last group is 1. No reversal occurs.\n</pre>\n\n<p><strong>Example 3:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2021/11/17/ex3.png\" style=\"width: 348px; height: 114px;\" />\n<pre>\n<strong>Input:</strong> head = [1,1,0,6,5]\n<strong>Output:</strong> [1,0,1,5,6]\n<strong>Explanation:</strong>\n- The length of the first group is 1. No reversal occurs.\n- The length of the second group is 2. The nodes are reversed.\n- The length of the last group is 2. The nodes are reversed.\n</pre>\n\n<p>&nbsp;</p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li>The number of nodes in the list is in the range <code>[1, 10<sup>5</sup>]</code>.</li>\n\t<li><code>0 &lt;= Node.val &lt;= 10<sup>5</sup></code></li>\n</ul>\n",
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            "categoryTitle": "Algorithms",
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                    "name": "Linked List",
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                    "lang": "C++",
                    "langSlug": "cpp",
                    "code": "/**\n * Definition for singly-linked list.\n * struct ListNode {\n *     int val;\n *     ListNode *next;\n *     ListNode() : val(0), next(nullptr) {}\n *     ListNode(int x) : val(x), next(nullptr) {}\n *     ListNode(int x, ListNode *next) : val(x), next(next) {}\n * };\n */\nclass Solution {\npublic:\n    ListNode* reverseEvenLengthGroups(ListNode* head) {\n        \n    }\n};",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "Java",
                    "langSlug": "java",
                    "code": "/**\n * Definition for singly-linked list.\n * public class ListNode {\n *     int val;\n *     ListNode next;\n *     ListNode() {}\n *     ListNode(int val) { this.val = val; }\n *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }\n * }\n */\nclass Solution {\n    public ListNode reverseEvenLengthGroups(ListNode head) {\n        \n    }\n}",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "Python",
                    "langSlug": "python",
                    "code": "# Definition for singly-linked list.\n# class ListNode(object):\n#     def __init__(self, val=0, next=None):\n#         self.val = val\n#         self.next = next\nclass Solution(object):\n    def reverseEvenLengthGroups(self, head):\n        \"\"\"\n        :type head: Optional[ListNode]\n        :rtype: Optional[ListNode]\n        \"\"\"\n        ",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "Python3",
                    "langSlug": "python3",
                    "code": "# Definition for singly-linked list.\n# class ListNode:\n#     def __init__(self, val=0, next=None):\n#         self.val = val\n#         self.next = next\nclass Solution:\n    def reverseEvenLengthGroups(self, head: Optional[ListNode]) -> Optional[ListNode]:\n        ",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "C",
                    "langSlug": "c",
                    "code": "/**\n * Definition for singly-linked list.\n * struct ListNode {\n *     int val;\n *     struct ListNode *next;\n * };\n */\n\n\nstruct ListNode* reverseEvenLengthGroups(struct ListNode* head){\n\n}",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "C#",
                    "langSlug": "csharp",
                    "code": "/**\n * Definition for singly-linked list.\n * public class ListNode {\n *     public int val;\n *     public ListNode next;\n *     public ListNode(int val=0, ListNode next=null) {\n *         this.val = val;\n *         this.next = next;\n *     }\n * }\n */\npublic class Solution {\n    public ListNode ReverseEvenLengthGroups(ListNode head) {\n        \n    }\n}",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "JavaScript",
                    "langSlug": "javascript",
                    "code": "/**\n * Definition for singly-linked list.\n * function ListNode(val, next) {\n *     this.val = (val===undefined ? 0 : val)\n *     this.next = (next===undefined ? null : next)\n * }\n */\n/**\n * @param {ListNode} head\n * @return {ListNode}\n */\nvar reverseEvenLengthGroups = function(head) {\n    \n};",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "Ruby",
                    "langSlug": "ruby",
                    "code": "# Definition for singly-linked list.\n# class ListNode\n#     attr_accessor :val, :next\n#     def initialize(val = 0, _next = nil)\n#         @val = val\n#         @next = _next\n#     end\n# end\n# @param {ListNode} head\n# @return {ListNode}\ndef reverse_even_length_groups(head)\n    \nend",
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                {
                    "lang": "Swift",
                    "langSlug": "swift",
                    "code": "/**\n * Definition for singly-linked list.\n * public class ListNode {\n *     public var val: Int\n *     public var next: ListNode?\n *     public init() { self.val = 0; self.next = nil; }\n *     public init(_ val: Int) { self.val = val; self.next = nil; }\n *     public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }\n * }\n */\nclass Solution {\n    func reverseEvenLengthGroups(_ head: ListNode?) -> ListNode? {\n        \n    }\n}",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "Go",
                    "langSlug": "golang",
                    "code": "/**\n * Definition for singly-linked list.\n * type ListNode struct {\n *     Val int\n *     Next *ListNode\n * }\n */\nfunc reverseEvenLengthGroups(head *ListNode) *ListNode {\n    \n}",
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                {
                    "lang": "Scala",
                    "langSlug": "scala",
                    "code": "/**\n * Definition for singly-linked list.\n * class ListNode(_x: Int = 0, _next: ListNode = null) {\n *   var next: ListNode = _next\n *   var x: Int = _x\n * }\n */\nobject Solution {\n    def reverseEvenLengthGroups(head: ListNode): ListNode = {\n        \n    }\n}",
                    "__typename": "CodeSnippetNode"
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                {
                    "lang": "Kotlin",
                    "langSlug": "kotlin",
                    "code": "/**\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 */\nclass Solution {\n    fun reverseEvenLengthGroups(head: ListNode?): ListNode? {\n        \n    }\n}",
                    "__typename": "CodeSnippetNode"
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                {
                    "lang": "Rust",
                    "langSlug": "rust",
                    "code": "// Definition for singly-linked list.\n// #[derive(PartialEq, Eq, Clone, Debug)]\n// pub struct ListNode {\n//   pub val: i32,\n//   pub next: Option<Box<ListNode>>\n// }\n// \n// impl ListNode {\n//   #[inline]\n//   fn new(val: i32) -> Self {\n//     ListNode {\n//       next: None,\n//       val\n//     }\n//   }\n// }\nimpl Solution {\n    pub fn reverse_even_length_groups(head: Option<Box<ListNode>>) -> Option<Box<ListNode>> {\n        \n    }\n}",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "PHP",
                    "langSlug": "php",
                    "code": "/**\n * Definition for a singly-linked list.\n * class ListNode {\n *     public $val = 0;\n *     public $next = null;\n *     function __construct($val = 0, $next = null) {\n *         $this->val = $val;\n *         $this->next = $next;\n *     }\n * }\n */\nclass Solution {\n\n    /**\n     * @param ListNode $head\n     * @return ListNode\n     */\n    function reverseEvenLengthGroups($head) {\n        \n    }\n}",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "TypeScript",
                    "langSlug": "typescript",
                    "code": "/**\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 reverseEvenLengthGroups(head: ListNode | null): ListNode | null {\n\n};",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "Racket",
                    "langSlug": "racket",
                    "code": "; Definition for singly-linked list:\n#|\n\n; val : integer?\n; next : (or/c list-node? #f)\n(struct list-node\n  (val next) #:mutable #:transparent)\n\n; constructor\n(define (make-list-node [val 0])\n  (list-node val #f))\n\n|#\n\n(define/contract (reverse-even-length-groups head)\n  (-> (or/c list-node? #f) (or/c list-node? #f))\n\n  )",
                    "__typename": "CodeSnippetNode"
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                {
                    "lang": "Erlang",
                    "langSlug": "erlang",
                    "code": "%% Definition for singly-linked list.\n%%\n%% -record(list_node, {val = 0 :: integer(),\n%%                     next = null :: 'null' | #list_node{}}).\n\n-spec reverse_even_length_groups(Head :: #list_node{} | null) -> #list_node{} | null.\nreverse_even_length_groups(Head) ->\n  .",
                    "__typename": "CodeSnippetNode"
                },
                {
                    "lang": "Elixir",
                    "langSlug": "elixir",
                    "code": "# Definition for singly-linked list.\n#\n# defmodule ListNode do\n#   @type t :: %__MODULE__{\n#           val: integer,\n#           next: ListNode.t() | nil\n#         }\n#   defstruct val: 0, next: nil\n# end\n\ndefmodule Solution do\n  @spec reverse_even_length_groups(head :: ListNode.t | nil) :: ListNode.t | nil\n  def reverse_even_length_groups(head) do\n\n  end\nend",
                    "__typename": "CodeSnippetNode"
                }
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            "hints": [
                "Consider the list structure ...A → (B → ... → C) → D..., where the nodes between B and C (inclusive) form a group, A is the last node of the previous group, and D is the first node of the next group. How can you utilize this structure?",
                "Suppose you have B → ... → C reversed (because it was of even length) so that it is now C → ... → B. What references do you need to fix so that the transitions between the previous, current, and next groups are correct?",
                "A.next should be set to C, and B.next should be set to D.",
                "Once the current group is finished being modified, you need to find the new A, B, C, and D nodes for the next group. How can you use the old A, B, C, and D nodes to find the new ones?",
                "The new A is either the old B or old C depending on if the group was of even or odd length. The new B is always the old D. The new C and D can be found based on the new B and the next group's length.",
                "You can set the initial values of A, B, C, and D to A = null, B = head, C = head, D = head.next. Repeat the steps from the previous hints until D is null."
            ],
            "solution": null,
            "status": null,
            "sampleTestCase": "[5,2,6,3,9,1,7,3,8,4]",
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