Solve problem: Merge Two Sorted Lists

Author: GolubevDS

README.MD

@@ -1,6 +1,6 @@
 # LEETCODE PATTERNS
 
-Solutions to problems from the [LeetCode Patterns](https://seanprashad.com/leetcode-patterns/) website
+Solving problems with LeetCode
 
 ## Problems
 
@@ -25,3 +25,4 @@ Solutions to problems from the [LeetCode Patterns](https://seanprashad.com/leetc
 | [Find Pivot Index](https://leetcode.com/problems/find-pivot-index/)                                                | [Solution](https://github.com/GolubevDS/LeetCodePatterns/blob/main/solutions/pivotIndex/index.js)             | Easy       |
 | [Is Subsequence](https://leetcode.com/problems/is-subsequence/)                                                    | [Solution](https://github.com/GolubevDS/LeetCodePatterns/blob/main/solutions/isSubsequence/isSubsequence.js)  | Easy       |
 | [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       |

helpers/createLinkedListFromArray.js

@@ -0,0 +1,24 @@
+function createLinkedListFromArray(arr) {
+	if (!arr || arr.length === 0) {
+		return null;
+	}
+
+	let head = {
+		val: arr[0],
+		next: null,
+	};
+	let current = head;
+
+	for (let i = 1; i < arr.length; i++) {
+		let newNode = {
+			val: arr[i],
+			next: null,
+		};
+		current.next = newNode;
+		current = newNode;
+	}
+
+	return head;
+}
+
+module.exports = createLinkedListFromArray;

solutions/isIsomorphic/isIsomorphic.spec.js

@@ -1,6 +1,6 @@
 const isIsomorphic = require('./isIsomorphic');
 
-describe('isIsomorphic', () => {
+describe('Isomorphic Strings', () => {
 	it('should return true when s and t are empty strings', () => {
 		const result = isIsomorphic('', '');
 		expect(result).toBe(true);

solutions/mergeTwoLists/README.MD

@@ -0,0 +1,7 @@
+## [21. Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/description/)
+
+You are given the heads of two sorted linked lists `list1` and `list2`.
+
+Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.
+
+Return the head of the merged linked list.

solutions/mergeTwoLists/mergeTwoLists.js

@@ -0,0 +1,23 @@
+/**
+ * @param {ListNode} list1
+ * @param {ListNode} list2
+ * @return {ListNode}
+ */
+function mergeTwoLists(list1, list2) {
+	if (!list1) return list2;
+	if (!list2) return list1;
+
+	const result = {};
+
+	if (list1.val < list2.val) {
+		result.val = list1.val;
+		result.next = mergeTwoLists(list1.next, list2);
+	} else {
+		result.val = list2.val;
+		result.next = mergeTwoLists(list1, list2.next);
+	}
+
+	return result;
+}
+
+module.exports = mergeTwoLists;

solutions/mergeTwoLists/mergeTwoLists.spec.js

@@ -0,0 +1,46 @@
+const createLinkedListFromArray = require('../../helpers/createLinkedListFromArray');
+const mergeTwoLists = require('./mergeTwoLists');
+
+describe('Merge Two Sorted Lists', () => {
+	it('should return null when both input lists are empty', () => {
+		const result = mergeTwoLists(null, null);
+		expect(result).toBeNull();
+	});
+
+	it('should return the non-empty list when one input list is empty', () => {
+		const l1 = createLinkedListFromArray([1, 2, 3]);
+		const result1 = mergeTwoLists(l1, null);
+		expect(result1).toEqual(l1);
+
+		const l2 = createLinkedListFromArray([4, 5, 6]);
+		const result2 = mergeTwoLists(null, l2);
+		expect(result2).toEqual(l2);
+	});
+
+	it('should return the merged list when both input lists are non-empty and sorted', () => {
+		const l1 = createLinkedListFromArray([1, 3, 5]);
+		const l2 = createLinkedListFromArray([2, 4, 6]);
+		const expected = createLinkedListFromArray([1, 2, 3, 4, 5, 6]);
+
+		const result = mergeTwoLists(l1, l2);
+		expect(result).toEqual(expected);
+	});
+
+	it('should handle edge cases where the input lists contain duplicate values', () => {
+		const l1 = createLinkedListFromArray([1, 2, 2, 3, 4, 5]);
+		const l2 = createLinkedListFromArray([1, 3, 4, 5, 6, 6]);
+		const expected = createLinkedListFromArray([1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6]);
+
+		const result = mergeTwoLists(l1, l2);
+		expect(result).toEqual(expected);
+	});
+
+	it('should handle edge cases where the input lists have different lengths', () => {
+		const l1 = createLinkedListFromArray([1, 3, 5]);
+		const l2 = createLinkedListFromArray([2, 4]);
+		const expected = createLinkedListFromArray([1, 2, 3, 4, 5]);
+
+		const result = mergeTwoLists(l1, l2);
+		expect(result).toEqual(expected);
+	});
+});