chore: add reverse to singly linked list (TheAlgorithms#1241)

* Fix: Reverse Singly Linked List

* Added reverse in SinglyLinkedList

* Added reverse in SinglyLinkedList

* Changes made

Author: Khushi-Shukla

Data-Structures/Linked-List/ReverseSinglyLinkedList.js

@@ -0,0 +1,16 @@
+/** A LinkedList based solution to reverse a number
+Problem Statement: Given a number such that each of its digit is stored in a singly linked list. Reverse the linked list and return the head of the linked list Link for the Problem: https://leetcode.com/problems/reverse-linked-list/ */
+class ReverseSinglyLinkedList {
+  solution (head) {
+    let prev = null
+    let next = null
+    while (head) {
+      next = head.next
+      head.next = prev
+      prev = head
+      head = next
+    }
+    return prev
+  };
+}
+export { ReverseSinglyLinkedList }

Data-Structures/Linked-List/SinglyLinkedList.js

@@ -274,6 +274,20 @@ class LinkedList {
   log () {
     console.log(JSON.stringify(this.headNode, null, 2))
   }
+
+  // Method to reverse the LinkedList
+  reverse () {
+    let head = this.headNode
+    let prev = null
+    let next = null
+    while (head) {
+      next = head.next
+      head.next = prev
+      prev = head
+      head = next
+    }
+    this.headNode = prev
+  };
 }
 
 export { Node, LinkedList }

Data-Structures/Linked-List/test/ReverseSinglyLinkedList.test.js

@@ -0,0 +1,14 @@
+import { ReverseSinglyLinkedList } from '../ReverseSinglyLinkedList'
+import { Node } from '../SinglyLinkedList'
+describe('ReverseSinglyLinkedList', () => {
+  it('Reverse a Number Represented as Linked List', () => {
+    const headNode = new Node(3)
+    headNode.next = new Node(4)
+    headNode.next.next = new Node(1)
+    const expected = new Node(1)
+    expected.next = new Node(4)
+    expected.next.next = new Node(3)
+    const reverseSinglyLinkedList = new ReverseSinglyLinkedList()
+    expect(reverseSinglyLinkedList.solution(headNode)).toEqual(expected)
+  })
+})

Data-Structures/Linked-List/test/SinglyLinkedList.test.js

@@ -247,4 +247,10 @@ describe('SinglyLinkedList', () => {
     headNode.rotateListRight(5)
     expect(headNode.get()).toEqual([20, 30, 40, 50, 10])
   })
+
+  it('Reverse a Linked List', () => {
+    const list = new LinkedList([4, 3, 1])
+    list.reverse()
+    expect(list.get()).toEqual([1, 3, 4])
+  })
 })