Singly Linked List – Reverse Operation

Reverse Operation in Singly Linked List

Reversing the singly linked list so that the last node becomes the new head and all next pointers are flipped.

Reversing a singly linked list involves reassigning the next pointer of each node to point to its previous node instead of the next one.

Since singly linked lists do not have prev pointers, we must carefully use extra pointers while traversing to track the previous and next nodes during reversal.

Visual Example

Suppose we have the following singly linked list:

We want to reverse the list so that the order becomes 50 → 40 → 30 → 20 → 10.

Step-by-Step Reversal

Start with prev = null and current = head (10).
Step 1: current = 10
Save next = 20, reverse the link (10 → null), move prev = 10, current = 20.
Step 2: current = 20
Save next = 30, reverse link (20 → 10), move prev = 20, current = 30.
Step 3: current = 30
Save next = 40, reverse link (30 → 20), move prev = 30, current = 40.
Step 4: current = 40
Save next = 50, reverse link (40 → 30), move prev = 40, current = 50.
Step 5: current = 50
Save next = null, reverse link (50 → 40), move prev = 50, current = null.

At the end, prev points to the new head of the reversed list, and current is null. Return prev as the new head.

Complete Pseudocode: Reverse a Singly Linked List

This pseudocode shows how to reverse a singly linked list in-place by manipulating the node pointers.

function reverse(head):
    prev = null
    curr = head

    while curr != null:
        next = curr.next
        curr.next = prev
        prev = curr
        curr = next

    return prev

Let’s understand each part of the code step by step.

function reverse(head):
    prev = null
    curr = head

Explanation: We initialize prev as null because the new tail will point to null. curr starts at the head of the list.

    while curr != null:
        next = curr.next
        curr.next = prev
        prev = curr
        curr = next

Explanation: We iterate through the list and reverse the direction of each link one by one. We use next to temporarily hold the next node before updating pointers.

    return prev

Explanation: After the loop, prev points to the new head of the reversed list, so we return it.