Day 1 - Reverse a linked list.md

Difficulty	Source	Tags
Easy	160 Days of Problem Solving	Linked-List

🚀 Day 1. Reverse a linked list 🧠

The problem can be found at the following link: Problem Link

💡 Problem Description:

You are given the head of a singly linked list. Your task is to reverse the list and return the reversed head.

🔍 Example Walkthrough:

Input:
head: 1 -> 2 -> 3 -> 4 -> NULL
Output:
head: 4 -> 3 -> 2 -> 1 -> NULL

Explanation:

Input:
head: 2 -> 7 -> 10 -> 9 -> 8 -> NULL
Output:
head: 8 -> 9 -> 10 -> 7 -> 2 -> NULL

Explanation:

Input:
head: 10 -> NULL
Output:
head: 10 -> NULL

Explanation:

Constraints:

• 1 <= number of nodes, data of nodes <= $10^5$

🎯 My Approach:

1. Iterative Reversal Algorithm:
   The problem can be efficiently solved using an iterative approach by traversing the linked list and reversing the next pointers of each node.

   • Start with two pointers: prev (initialized to NULL) and current (initialized to the head of the list).
   • In each iteration, update the next pointer of the current node to point to the prev node, then move prev and current forward.
   • Continue the process until the entire list is reversed.

2. Steps:

   • Initialize prev to NULL and current to the head of the list.
   • Traverse the list while current is not NULL.
   • For each node, reverse the next pointer to point to prev.
   • Move the prev and current pointers one step forward.
   • Once the list is completely reversed, return prev as the new head.

🕒 Time and Auxiliary Space Complexity

• Expected Time Complexity: O(n), where n is the number of nodes in the linked list. We traverse the entire list once.
• Expected Auxiliary Space Complexity: O(1), as we only use a constant amount of additional space (for the prev and current pointers).

📝 Solution Code

Code (C++)

class Solution {
public:
    Node* reverseList(Node* head) {
        Node *prev = NULL, *curr = head, *next;
        while (curr) {
            next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }
};

👨‍💻 Alternative Approach

Pointer Manipulation Using swap for Linked List Reversal

Explanation:

This approach leverages pointer manipulation to reverse the linked list in place, using the swap function to reduce the verbosity of code and streamline pointer updates. The key idea is to iteratively reverse the next pointers of each node while traversing the list.

Detailed Steps:

1. Initialization:

   • prev is initialized to nullptr (to mark the new end of the reversed list).
   • head starts as the current node in the original list.

2. Iterative Reversal:

   • In each iteration of the loop, two swaps are used to reassign pointers:
     • Swap head->next with prev: This updates the current node's next pointer to point to the previous node.
     • Swap head with prev: This shifts the prev pointer to the current node (marking progress in the reversed list) and moves the head pointer forward to the next node in the original list.

3. Termination:

   • The loop continues until head becomes nullptr (indicating the end of the list has been reached).
   • At this point, prev holds the new head of the reversed linked list.

4. Return Value:

   • The method returns prev, which now points to the reversed list's head.

Solution

class Solution {
public:
    Node* reverseList(Node* head) {
        Node *prev = nullptr;
        while (head) {
            swap(head->next, prev), swap(head, prev);
        }
        return prev;
    }
};

Key Characteristics:

• Runtime Efficiency:
  • The approach has a time complexity of (O(n)), where (n) is the number of nodes in the list. Each node is processed exactly once.
• Space Efficiency:
  • The space complexity is (O(1)) as no additional memory is used apart from a few pointers.

Example Walkthrough:

Let the initial linked list be: $(1 \rightarrow 2 \rightarrow 3 \rightarrow \text{nullptr})$

1. Initialization: prev = nullptr, head = 1
2. First Iteration:
   • Swap head->next and prev: Now 1->next = nullptr
   • Swap head and prev: prev = 1, head = 2
3. Second Iteration:
   • Swap head->next and prev: Now 2->next = 1
   • Swap head and prev: prev = 2, head = 3
4. Third Iteration:
   • Swap head->next and prev: Now 3->next = 2
   • Swap head and prev: prev = 3, head = nullptr
5. Termination:
   • head is nullptr, and prev points to the reversed list: $(3 \rightarrow 2 \rightarrow 1 \rightarrow \text{nullptr})$

Code (Java)

class Solution {
    Node reverseList(Node head) {
        Node prev = null;
        while (head != null) {
            Node next = head.next;
            head.next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }
}

Code (Python)

class Solution:
    def reverseList(self, head):
        prev = None
        while head:
            next = head.next
            head.next = prev
            prev = head
            head = next
        return prev

🎯 Contribution and Support:

For discussions, questions, or doubts related to this solution, feel free to connect on LinkedIn: Any Questions. Let’s make this learning journey more collaborative!

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