Feat | NeetCode | RoadMap | #34 Reversed Linked List

README.md

@@ -107,6 +107,8 @@
   - [Task #31](src/neetcode/binary-search/search-in-rotated-sorted-array) – _Search In Rotated Sorted Array_ - _[check the task](https://neetcode.io/problems/find-target-in-rotated-sorted-array)_
   - [Task #32](src/neetcode/binary-search/time-based-key-value-store) – _Time Based Key Value Store_ - _[check the task](https://neetcode.io/problems/time-based-key-value-store)_
   - [Task #33](src/neetcode/binary-search/median-of-two-sorted-arrays) – _Median Of Two Sorted Arrays_ - _[check the task](https://neetcode.io/problems/median-of-two-sorted-arrays)_
+- Linked List
+  - [Task #34](src/neetcode/linked-list/reversed-linked-list) – _Reversed Linked List_ - _[check the task](https://neetcode.io/problems/reverse-a-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/reversed-linked-list/solution.test.ts

@@ -0,0 +1,59 @@
+import { ListNode, reverseList } from './solution';
+
+const createList = (values: number[]): ListNode | null => {
+  const dummy = new ListNode();
+  let tail = dummy;
+
+  for (const value of values) {
+    tail.next = new ListNode(value);
+    tail = tail.next;
+  }
+
+  return dummy.next;
+};
+
+const listToArray = (head: ListNode | null): number[] => {
+  const values: number[] = [];
+  let current = head;
+
+  while (current) {
+    values.push(current.val);
+    current = current.next;
+  }
+
+  return values;
+};
+
+describe('Reversed Linked List | NeetCode | RoadMap | Testcases', () => {
+  test('#1 Reverses a linked list', () => {
+    const head = createList([0, 1, 2, 3]);
+
+    expect(listToArray(reverseList(head))).toEqual([3, 2, 1, 0]);
+  });
+
+  test('returns null for an empty list', () => {
+    expect(reverseList(null)).toBeNull();
+  });
+
+  test('#2 Returns the same node for a single-node list', () => {
+    const head = new ListNode(42);
+
+    expect(reverseList(head)).toBe(head);
+    expect(head.next).toBeNull();
+  });
+
+  test('#3 Reverses the existing links instead of creating new nodes', () => {
+    const first = new ListNode(1);
+    const second = new ListNode(2);
+    const third = new ListNode(3);
+    first.next = second;
+    second.next = third;
+
+    const reversed = reverseList(first);
+
+    expect(reversed).toBe(third);
+    expect(third.next).toBe(second);
+    expect(second.next).toBe(first);
+    expect(first.next).toBeNull();
+  });
+});

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

@@ -0,0 +1,26 @@
+export class ListNode {
+  val: number;
+  next: ListNode | null;
+  constructor(val = 0, next = null) {
+    this.val = val;
+    this.next = next;
+  }
+}
+
+/**
+ * @param {ListNode} head - the beginning of a singly linked list
+ * @return {ListNode} - the new beginning of the reversed list.
+ */
+export const reverseList = (head: ListNode | null): ListNode | null => {
+  let prev: ListNode | null = null;
+  let curr = head;
+
+  while (curr !== null) {
+    const temp = curr.next;
+    curr.next = prev;
+    prev = curr;
+    curr = temp;
+  }
+
+  return prev;
+};