Solve problem: N-ary Tree Preorder Traversal

Author: GolubevDS

README.MD

@@ -27,4 +27,5 @@ Solving problems with LeetCode
 | [Isomorphic Strings](https://leetcode.com/problems/isomorphic-strings/)                                            | [Solution](https://github.com/GolubevDS/LeetCodePatterns/blob/main/solutions/isIsomorphic/isIsomorphic.js)           | Easy       |
 | [Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)                                    | [Solution](https://github.com/GolubevDS/LeetCodePatterns/blob/main/solutions/mergeTwoLists/mergeTwoLists.js)         | Easy       |
 | [Longest Palindrome](https://leetcode.com/problems/longest-palindrome/)                                            | [Solution](https://github.com/GolubevDS/LeetCodePatterns/blob/main/solutions/longestPalindrome/longestPalindrome.js) | Easy       |
+| [N-ary Tree Preorder Traversal](https://leetcode.com/problems/n-ary-tree-preorder-traversal/)                      | [Solution](https://github.com/GolubevDS/LeetCodePatterns/blob/main/solutions/preorder/preorder.js)                   | Easy       |
 | [Linked List Cycle II](https://leetcode.com/problems/linked-list-cycle-ii/description/)                            | [Solution](https://github.com/GolubevDS/LeetCodePatterns/blob/main/solutions/detectCycle/detectCycle.js)             | Medium     |

solutions/index.d.ts

@@ -3,3 +3,9 @@ declare class ListNode {
 	next: ListNode | null;
 	constructor(val: number);
 }
+// @ts-ignore
+declare class Node {
+	val: number;
+	children: Node[];
+	constructor(val: number, children: Node[]);
+}

solutions/preorder/README.MD

@@ -0,0 +1,5 @@
+## [589. N-ary Tree Preorder Traversal](https://leetcode.com/problems/n-ary-tree-preorder-traversal/)
+
+Given the `root` of an n-ary tree, return the preorder traversal of its nodes' values.
+
+Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)

solutions/preorder/preorder.js

@@ -0,0 +1,40 @@
+/**
+ * @param {Node|null} root
+ * @return {number[]}
+ */
+function preorder(root) {
+	if (!root) return [];
+
+	const stack = [root];
+	const result = [];
+
+	while (stack.length > 0) {
+		const node = stack.pop();
+		result.push(node.val);
+		for (let i = node.children.length - 1; i >= 0; i--) {
+			stack.push(node.children[i]);
+		}
+	}
+
+	return result;
+}
+
+// function preorder(root) {
+// 	if (!root) return []
+//
+// 	const queue = root.children;
+// 	const result = [root.val];
+//
+// 	while (queue.length > 0) {
+// 		const node = queue.shift();
+// 		result.push(node.val);
+//
+// 		if (node.children.length > 0) {
+// 			queue.unshift(...node.children);
+// 		}
+// 	}
+//
+// 	return result;
+// }
+
+module.exports = preorder;

solutions/preorder/preorder.spec.js

@@ -0,0 +1,42 @@
+const preorder = require('./preorder');
+
+class Node {
+	val;
+	children;
+
+	constructor(val, children) {
+		this.val = val;
+		this.children = children;
+	}
+}
+
+function createTreeRoot() {
+	return new Node(1, [
+		new Node(2, [
+			new Node(4, []),
+			new Node(5, [])
+		]),
+		new Node(3, [
+			new Node(6, []),
+			new Node(7, [
+				new Node(8, [])
+			])
+		])
+	]);
+}
+
+describe('N-ary Tree Preorder Traversal', () => {
+	test('should return an empty array when root is null', () => {
+		expect(preorder(null)).toEqual([]);
+	});
+
+	test('should return an array of values in preorder traversal order', () => {
+		const root = createTreeRoot();
+		expect(preorder(root)).toEqual([1, 2, 4, 5, 3, 6, 7, 8]);
+	});
+
+	test('should return an array with a single value when root has no children', () => {
+		const root = new Node(1, []);
+		expect(preorder(root)).toEqual([1]);
+	});
+});