Linked Lists: Middle of the Linked List

Author: jhonnymoreira

Diff for: README.md

@@ -20,7 +20,7 @@
   - [x] [Jewels and Stones](./src/hashing/jewels-and-stones.ts)
   - [x] [Longest Substring Without Repeating Characters](./src/hashing/longest-substring-without-repeating-characters.ts)
 - [ ] Linked Lists
-  - [ ] [Middle of the Linked List](./src/linked-lists/middle-of-the-linked-list.ts)
+  - [x] [Middle of the Linked List](./src/linked-lists/middle-of-the-linked-list.ts)
   - [ ] [Remove Duplicates from Sorted List](./src/linked-lists/remove-duplicates-from-sorted-list.ts)
   - [ ] [Reverse Linked List II](./src/linked-lists/reverse-linked-list-ii.ts)
 - [ ] Stacks and Queues

Diff for: src/linked-lists/middle-of-the-linked-list.ts

@@ -1 +1,20 @@
-export function () {}
+export class ListNode {
+  val: number;
+  next: ListNode | null;
+  constructor(val?: number, next?: ListNode | null) {
+    this.val = val === undefined ? 0 : val;
+    this.next = next === undefined ? null : next;
+  }
+}
+
+export function middleNode(head: ListNode | null): ListNode | null {
+  let slow = head;
+  let fast = head;
+
+  while (fast && fast.next) {
+    slow = slow!.next;
+    fast = fast.next.next;
+  }
+
+  return slow;
+}

Diff for: tests/linked-lists/middle-of-the-linked-list.test.ts

@@ -1,3 +1,31 @@
-import {} from '@/linked-lists/middle-of-the-linked-list.js';
+import {
+  ListNode,
+  middleNode,
+} from '@/linked-lists/middle-of-the-linked-list.js';
 
-describe.todo('Linked Lists: Middle of the Linked List')
+describe('Linked Lists: Middle of the Linked List', () => {
+  test.each([
+    {
+      head: new ListNode(
+        1,
+        new ListNode(2, new ListNode(3, new ListNode(4, new ListNode(5, null))))
+      ),
+      output: new ListNode(3, new ListNode(4, new ListNode(5, null))),
+    },
+    {
+      head: new ListNode(
+        1,
+        new ListNode(
+          2,
+          new ListNode(
+            3,
+            new ListNode(4, new ListNode(5, new ListNode(6, null)))
+          )
+        )
+      ),
+      output: new ListNode(4, new ListNode(5, new ListNode(6, null))),
+    },
+  ])('middleNode($head) === $output', ({ head, output }) => {
+    expect(middleNode(head)).toStrictEqual(output);
+  });
+});