Feat | NeetCode | Roadmap | #37 Reorder List

README.md

@@ -111,6 +111,7 @@
   - [Task #34](src/neetcode/linked-list/reversed-linked-list) – _Reversed Linked List_ - _[check the task](https://neetcode.io/problems/reverse-a-linked-list)_
   - [Task #35](src/neetcode/linked-list/merge-two-sorted-lists) – _Merge Two Sorted Lists_ - _[check the task](https://neetcode.io/problems/merge-two-sorted-linked-lists)_
   - [Task #36](src/neetcode/linked-list/linked-list-cycle) – _Linked List Cycle_ - _[check the task](https://neetcode.io/problems/linked-list-cycle-detection)_
+  - [Task #37](src/neetcode/linked-list/reorder-list) – _Reorder List_ - _[check the task](https://neetcode.io/problems/reorder-linked-list)_
 
 [BigFrontEnd - JavaScript Coding Questions](https://bigfrontend.dev/problem)
 - [1. implement curry()](src/bigfrontend/javascript/curry) - _Curry Function_ - [check the task](https://bigfrontend.dev/problem/implement-curry)

src/neetcode/linked-list/reorder-list/solution.test.ts

@@ -0,0 +1,25 @@
+import { createList, listToArray } from '../common';
+import { reorderList } from './solution';
+
+describe('Reorder List | NeetCode | RoadMap | Testcases', () => {
+  test('#1', () => {
+    const head = createList([2, 4, 6, 8]);
+    const expected = [2, 8, 4, 6];
+    reorderList(head);
+    expect(listToArray(head)).toEqual(expected);
+  });
+
+  test('#2', () => {
+    const head = createList([2, 4, 6, 8, 10]);
+    const expected = [2, 10, 4, 8, 6];
+    reorderList(head);
+    expect(listToArray(head)).toEqual(expected);
+  });
+
+  test('#3', () => {
+    const head = createList([]);
+    const expected: number[] = [];
+    reorderList(head);
+    expect(listToArray(head)).toEqual(expected);
+  });
+});

src/neetcode/linked-list/reorder-list/solution.ts

@@ -0,0 +1,50 @@
+import { ListNode } from '../common';
+
+/**
+ * If a given linked list with head `head` of `length = n` is represented as:
+ *
+ * `[0, 1, ..., n - 2, n - 1]`
+ *
+ * Then function reorders its nodes to be in the following order:
+ *
+ * `[0, n-1, 1, n-2, 2, n-3, ...]`
+ *
+ * @param {ListNode} head - the head of a singly linked-list
+ * @return {void}
+ */
+export const reorderList = (head: ListNode | null): void => {
+  let slow: ListNode | null = head;
+  let fast: ListNode | null = head;
+
+  while (fast !== null && fast.next !== null) {
+    slow = slow?.next ?? null;
+    fast = fast.next.next;
+  }
+
+  let prev: ListNode | null = null;
+  let curr: ListNode | null = slow;
+
+  while (curr !== null) {
+    const next = curr.next;
+    curr.next = prev;
+    prev = curr;
+    curr = next;
+  }
+
+  let l1 = head;
+  let l2 = prev;
+
+  while (l2 !== null && l1 !== null) {
+    const nextF = l1.next;
+    const nextS = l2.next;
+    l1.next = l2;
+    l2.next = nextF;
+
+    l1 = nextF;
+    l2 = nextS;
+  }
+
+  if (l1 !== null) {
+    l1.next = null;
+  }
+};