JavaScript Program to Reverse a Linked List (original) (raw)

Last Updated : 05 Mar, 2024

A **linked list is a data structure where elements are stored in nodes, and each node points to the next node in the sequence. Reversing a **linked list involves changing the direction of the pointers, making the last node the new head and the head the new tail.

Below are the approaches to reverse a linked list in JavaScript:

Table of Content

**Reversing a Linked List Using Iteration

We will use a loop to iterate through the elements and the pointer nodes of the **linked list and change the next pointers of each node in such a way that their order is reversed.

**Example: The below code iteratively reverses the **linked list in **JavaScript.

JavaScript `

class Node { constructor(data, next = null) { this.data = data; this.next = next; } }

class LinkedList { constructor() { this.head = null; }

insertFirst(data) {
    this.head = new Node(data, this.head);
}

printList() {
    let current = this.head;
    while (current !== null) {
        console.log(current.data);
        current = current.next;
    }
}
reverse() {
    let prev = null;
    let current = this.head;
    let next = null;

    while (current !== null) {
        next = current.next;
        current.next = prev;
        prev = current;
        current = next;
    }
    this.head = prev;
}

}

const linkedList = new LinkedList();

linkedList.insertFirst(3); linkedList.insertFirst(2); linkedList.insertFirst(1);

console.log("Original Linked List:"); linkedList.printList();

linkedList.reverse();

console.log("\nReversed Linked List:"); linkedList.printList();

`

Output

Original Linked List: 1 2 3

Reversed Linked List: 3 2 1

**Time Complexity: O(N)

**Space Complexity: O(1)

Reversing a Linked List Using Recursion

**Recursion is a process in which a function calls itself directly or indirectly. We are going to reverse the **linked list by iterating through its elements using **recursion and change their node values.

**Example: The below code is the implementation of the recursive approach to reverse the **linked list in **JavaScript.

JavaScript `

class Node { constructor(data, next = null) { this.data = data; this.next = next; } }

class LinkedList { constructor() { this.head = null; }

insertFirst(data) {
    this.head = new Node(data, this.head);
}

reverseRecursiveUtil(current, prev) {
    if (current === null) {
        this.head = prev;
        return;
    }

    const next = current.next;
    current.next = prev;

    this.reverseRecursiveUtil(next, current);
}

reverseRecursive() {
    this.reverseRecursiveUtil(this.head, null);
}

printList() {
    let current = this.head;
    while (current !== null) {
        console.log(current.data);
        current = current.next;
    }
}

}

const linkedList = new LinkedList();

linkedList.insertFirst(3); linkedList.insertFirst(2); linkedList.insertFirst(1);

console.log("Original Linked List:"); linkedList.printList();

linkedList.reverseRecursive();

console.log("\nReversed Linked List:"); linkedList.printList();

`

Output

Original Linked List: 1 2 3

Reversed Linked List: 3 2 1

**Time Complexity: O(N)

**Space Complexity: O(N)

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