feat: add algorithm to merge two sorted linked list

DIRECTORY.md

@@ -61,6 +61,7 @@
   * Linked-List
     * [CycleDetection](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Linked-List/CycleDetection.js)
     * [DoublyLinkedList](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Linked-List/DoublyLinkedList.js)
+    * [MergeTwoSortedLinkedList](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Linked-List/MergeTwoSortedLinkedList.js)
     * [RotateListRight](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Linked-List/RotateListRight.js)
     * [SingleCircularLinkedList](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Linked-List/SingleCircularLinkedList.js.js)
     * [SinglyLinkList](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Linked-List/SinglyLinkList.js)

Data-Structures/Linked-List/MergeTwoSortedLinkedList.js

@@ -0,0 +1,35 @@
+// Problem source and explanation: https://leetcode.com/problems/merge-two-sorted-lists/
+
+/**
+ * Merge two sorted linked lists and return it as a sorted list. The list should be made by splicing together the nodes of the first two lists.
+ * @param {ListNode} l1
+ * @param {ListNode} l2
+ * @return {ListNode}
+ */
+
+const mergeTwoSortedLists = (l1, l2) => {
+  let node = l1
+  if (!l1) return l2
+  if (!l2) return l1
+  while (node.next) {
+    node = node.next
+  }
+  node.next = l2
+  node = l1
+  while (node) {
+    let curr = node
+    while (curr) {
+      if (node.val > curr.val) {
+        ;[curr.val, node.val] = [node.val, curr.val]
+      }
+      curr = curr.next
+    }
+    node = node.next
+  }
+  return l1
+}
+
+// console.log(mergeTwoLists([1, 2, 4], [1, 3, 4]))
+// console.log(mergeTwoLists([], [0]))
+// console.log(mergeTwoLists([], []))
+export { mergeTwoSortedLists }