N-ary Tree Preorder Traversal
<p>Given the <code>root</code> of an n-ary tree, return <em>the preorder traversal of its nodes&#39; values</em>.</p>

<p>Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<p><img src="https://assets.leetcode.com/uploads/2018/10/12/narytreeexample.png" style="width: 100%; max-width: 300px;" /></p>

<pre>
<strong>Input:</strong> root = [1,null,3,2,4,null,5,6]
<strong>Output:</strong> [1,3,5,6,2,4]
</pre>

<p><strong class="example">Example 2:</strong></p>

<p><img alt="" src="https://assets.leetcode.com/uploads/2019/11/08/sample_4_964.png" style="width: 296px; height: 241px;" /></p>

<pre>
<strong>Input:</strong> root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
<strong>Output:</strong> [1,2,3,6,7,11,14,4,8,12,5,9,13,10]
</pre>

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

<ul>
	<li>The number of nodes in the tree is in the range <code>[0, 10<sup>4</sup>]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 10<sup>4</sup></code></li>
	<li>The height of the n-ary tree is less than or equal to <code>1000</code>.</li>
</ul>

<p>&nbsp;</p>
<p><strong>Follow up:</strong> Recursive solution is trivial, could you do it iteratively?</p>

['binary-tree-preorder-traversal', 'n-ary-tree-level-order-traversal', 'n-ary-tree-postorder-traversal']
[]
[{"value": "cpp", "text": "C++", "defaultCode": "/*\n// Definition for a Node.\nclass Node {\npublic:\n    int val;\n    vector<Node*> children;\n\n    Node() {}\n\n    Node(int _val) {\n        val = _val;\n    }\n\n    Node(int _val, vector<Node*> _children) {\n        val = _val;\n        children = _children;\n    }\n};\n*/\n\nclass Solution {\npublic:\n    vector<int> preorder(Node* root) {\n        \n    }\n};"}, {"value": "java", "text": "Java", "defaultCode": "/*\n// Definition for a Node.\nclass Node {\n    public int val;\n    public List<Node> children;\n\n    public Node() {}\n\n    public Node(int _val) {\n        val = _val;\n    }\n\n    public Node(int _val, List<Node> _children) {\n        val = _val;\n        children = _children;\n    }\n};\n*/\n\nclass Solution {\n    public List<Integer> preorder(Node root) {\n        \n    }\n}"}, {"value": "python", "text": "Python", "defaultCode": "\"\"\"\n# Definition for a Node.\nclass Node(object):\n    def __init__(self, val=None, children=None):\n        self.val = val\n        self.children = children\n\"\"\"\n\nclass Solution(object):\n    def preorder(self, root):\n        \"\"\"\n        :type root: Node\n        :rtype: List[int]\n        \"\"\"\n        "}, {"value": "python3", "text": "Python3", "defaultCode": "\"\"\"\n# Definition for a Node.\nclass Node:\n    def __init__(self, val: Optional[int] = None, children: Optional[List['Node']] = None):\n        self.val = val\n        self.children = children\n\"\"\"\n\nclass Solution:\n    def preorder(self, root: 'Node') -> List[int]:\n        "}, {"value": "c", "text": "C", "defaultCode": "/**\n * Definition for a Node.\n * struct Node {\n *     int val;\n *     int numChildren;\n *     struct Node** children;\n * };\n */\n\n/**\n * Note: The returned array must be malloced, assume caller calls free().\n */\nint* preorder(struct Node* root, int* returnSize) {\n    \n}"}, {"value": "csharp", "text": "C#", "defaultCode": "/*\n// Definition for a Node.\npublic class Node {\n    public int val;\n    public IList<Node> children;\n\n    public Node() {}\n\n    public Node(int _val) {\n        val = _val;\n    }\n\n    public Node(int _val,IList<Node> _children) {\n        val = _val;\n        children = _children;\n    }\n}\n*/\n\npublic class Solution {\n    public IList<int> Preorder(Node root) {\n        \n    }\n}"}, {"value": "javascript", "text": "JavaScript", "defaultCode": "/**\n * // Definition for a _Node.\n * function _Node(val, children) {\n *    this.val = val;\n *    this.children = children;\n * };\n */\n\n/**\n * @param {_Node|null} root\n * @return {number[]}\n */\nvar preorder = function(root) {\n    \n};"}, {"value": "typescript", "text": "TypeScript", "defaultCode": "/**\n * Definition for _Node.\n * class _Node {\n *     val: number\n *     children: _Node[]\n * \n *     constructor(val?: number, children?: _Node[]) {\n *         this.val = (val===undefined ? 0 : val)\n *         this.children = (children===undefined ? [] : children)\n *     }\n * }\n */\n\n\nfunction preorder(root: _Node | null): number[] {\n\n};"}, {"value": "php", "text": "PHP", "defaultCode": "/**\n * Definition for a Node.\n * class Node {\n *     public $val = null;\n *     public $children = null;\n *     function __construct($val = 0) {\n *         $this->val = $val;\n *         $this->children = array();\n *     }\n * }\n */\n\nclass Solution {\n    /**\n     * @param Node $root\n     * @return integer[]\n     */\n    function preorder($root) {\n        \n    }\n}"}, {"value": "swift", "text": "Swift", "defaultCode": "/**\n * Definition for a Node.\n * public class Node {\n *     public var val: Int\n *     public var children: [Node]\n *     public init(_ val: Int) {\n *         self.val = val\n *         self.children = []\n *     }\n * }\n */\n\nclass Solution {\n    func preorder(_ root: Node?) -> [Int] {\n    \t\n    }\n}"}, {"value": "kotlin", "text": "Kotlin", "defaultCode": "/**\n * Definition for a Node.\n * class Node(var `val`: Int) {\n *     var children: List<Node?> = listOf()\n * }\n */\n\nclass Solution {\n    fun preorder(root: Node?): List<Int> {\n        \n    }\n}"}, {"value": "golang", "text": "Go", "defaultCode": "/**\n * Definition for a Node.\n * type Node struct {\n *     Val int\n *     Children []*Node\n * }\n */\n\nfunc preorder(root *Node) []int {\n    \n}"}, {"value": "ruby", "text": "Ruby", "defaultCode": "# Definition for a Node.\n# class Node\n#     attr_accessor :val, :children\n#     def initialize(val)\n#         @val = val\n#         @children = []\n#     end\n# end\n\n# @param {Node} root\n# @return {List[int]}\ndef preorder(root)\n    \nend"}, {"value": "scala", "text": "Scala", "defaultCode": "/**\n * Definition for a Node.\n * class Node(var _value: Int) {\n *   var value: Int = _value\n *   var children: List[Node] = List()\n * }\n */\n\nobject Solution {\n    def preorder(root: Node): List[Int] = {\n        \n    }\n}"}]
{
  "name": "preorder",
  "params": [
    {
      "name": "root",
      "type": "integer"
    }
  ],
  "return": {
    "type": "list<integer>"
  },
  "languages": [
    "cpp",
    "java",
    "python",
    "csharp",
    "javascript",
    "python3",
    "golang",
    "ruby",
    "c",
    "kotlin",
    "scala",
    "php",
    "swift",
    "typescript"
  ],
  "manual": true, "typescriptCustomType" : "class _Node {\n    val: number\n    children: _Node[]\n\n    constructor(val?: number, children?: _Node[]) {\n        this.val = (val===undefined ? 0 : val)\n        this.children = (children===undefined ? [] : children)\n    }\n}\n"

}
{"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>"]}
['Stack', 'Tree', 'Depth-First Search']