🔄 Reverse Linked List Algorithm

Learn to reverse a linked list using the three-pointer approach with interactive visualization and step-by-step animation.

📋 Problem Statement

Given the head of a singly linked list, reverse the list and return the reversed list.

Example:
Input: 1 → 2 → 3 → 4 → 5 → null
Output: 5 → 4 → 3 → 2 → 1 → null

🎯 Interactive Visualization

Step 0: Initialize three pointers

prev

curr

next

🧠 Three Pointer Approach

🔴 prev

Points to the previous node (initially null)

🔵 curr

Points to the current node being processed

🟢 next

Temporarily stores the next node

💻 Code Implementation

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */

function reverseList(head) {
    // Initialize three pointers
    let prev = null;
    let curr = head;
    
    // Traverse the list
    while (curr !== null) {
        // Store the next node
        let next = curr.next;
        
        // Reverse the link
        curr.next = prev;
        
        // Move pointers forward
        prev = curr;
        curr = next;
    }
    
    // prev is now the new head
    return prev;
}

📝 Step-by-Step Process

Step 1: Initialize

Set prev = null, curr = head. The prev pointer starts as null since there's no previous node.

Step 2: Store Next

Before breaking the link, save curr.next in the next pointer to avoid losing the rest of the list.

Step 3: Reverse Link

Point curr.next to prev, effectively reversing the direction of the current link.

Step 4: Move Forward

Advance prev to curr and curr to next, preparing for the next iteration.

Step 5: Repeat

Continue until curr becomes null. At this point, prev points to the new head of the reversed list.

⏱️ Time Complexity

O(n)

We visit each node exactly once

💾 Space Complexity

O(1)

Only three pointers used

💡 Practice Tips

✏️

Draw It Out

Visualize each step on paper

🧪

Test Edge Cases

Empty list, single node

🔄

Practice Iteratively

Master the pointer movement

🎯

Understand the Logic

Why three pointers are needed

🌟 Master the three-pointer technique and you'll have a powerful tool for many linked list problems!

Reverse Linked List - Three Pointer Algorithm