feat: add Reorder List

Author: dukeluo

README.md

@@ -39,6 +39,7 @@ https://neetcode.io/roadmap
 | 567 | [Permutation in String](https://leetcode.com/problems/permutation-in-string/)                                                   | Medium     | [ts](./TypeScript/567.permutation-in-string.ts)                                               | Sliding Window   |
 | 206 | [Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/)                                                       | Easy       | [ts](./TypeScript/206.reverse-linked-list.ts) [rkt](./Racket/206.reverse-linked-list.rkt)     | Linked List      |
 | 21  | [Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)                                                 | Easy       | [ts](./TypeScript/21.merge-two-sorted-lists.ts) [rkt](./Racket/21.merge-two-sorted-lists.rkt) | Linked List      |
+| 143 | [Reorder List](https://leetcode.com/problems/reorder-list/)                                                                     | Medium     | [ts](./TypeScript/143.reorder-list.ts)                                                        | Linked List      |
 
 ### Others
 

TypeScript/143.reorder-list.ts

@@ -0,0 +1,43 @@
+interface ListNode {
+  val: number;
+  next: ListNode | null;
+}
+
+function reorderList(head: ListNode | null): void {
+  let slow: ListNode | null | undefined = head;
+  let fast: ListNode | null | undefined = head?.next;
+  let midpoint: ListNode | null | undefined = null;
+
+  // get midpoint
+  while (fast && fast.next) {
+    slow = slow?.next;
+    fast = fast.next.next;
+  }
+  if (!slow?.next) return;
+  midpoint = slow.next;
+  slow.next = null;
+
+  // reverse
+  let reversed: ListNode | null = null;
+  while (midpoint) {
+    let next = midpoint.next;
+
+    midpoint.next = reversed;
+    reversed = midpoint;
+    midpoint = next;
+  }
+
+  // merge
+  let n1 = head;
+  let n2 = reversed;
+  while (n1 && n2) {
+    let next1 = n1.next;
+    let next2 = n2.next;
+
+    n1.next = n2;
+    n2.next = next1;
+
+    n1 = next1;
+    n2 = next2;
+  }
+}