Fix LinkedList.insert not updating the tail when inserting after the last node

src/data-structures/linked-list/LinkedList.js

@@ -75,6 +75,12 @@ export default class LinkedList {
       if (currentNode) {
         newNode.next = currentNode.next;
         currentNode.next = newNode;
+
+        // If the new node was linked after the current tail, it is the new tail.
+        // Otherwise the next append() would overwrite it and drop the node.
+        if (currentNode === this.tail) {
+          this.tail = newNode;
+        }
       } else {
         if (this.tail) {
           this.tail.next = newNode;

src/data-structures/linked-list/__test__/LinkedList.test.js

@@ -47,6 +47,22 @@ describe('LinkedList', () => {
     expect(linkedList.toString()).toBe('1,4,2,3,10');
   });
 
+  it('should update the tail when inserting after the last node', () => {
+    const linkedList = new LinkedList();
+
+    linkedList.append(1).append(2);
+    linkedList.insert(3, 2);
+
+    expect(linkedList.tail.value).toBe(3);
+    expect(linkedList.toString()).toBe('1,2,3');
+
+    linkedList.append(4);
+
+    expect(linkedList.toString()).toBe('1,2,3,4');
+    expect(linkedList.deleteTail().value).toBe(4);
+    expect(linkedList.deleteTail().value).toBe(3);
+  });
+
   it('should delete node by value from linked list', () => {
     const linkedList = new LinkedList();