how to reverse a list with O(1) space and O(n) time?

Question

I am looking for a method that reverses the same instance of a given list, with O(1) additional space and O(n) time.
this is not HW nor I am looking for some library me

Answer 1

Here is a solution in Java, with O(n) time complexity (just a single pass) and O(1) space complexity (Using just two temporary variables):

    private static void reverseLinkedList() {//O(n) Time complexity, O(1) space complexity 

    //two temp pointers
    Node next = null, previous = null;

    while(head.next != null){
        next = head.next;//move next to next address
        head.next = previous;   //previous node will be the next node for head, so that head will point reverse         
        previous = head; //incrementing previous to the current node
        head = next; //incrementing head 

    }
    //at this point head points to last node and previous has the remaining reversed array
    head.next = previous;
    System.out.println("\nReversed");

}

Full code goes like:

  package com.test;

public class LinkedListReverse {
    private static Node  head;
    public static void main(String[] args) {

        for(int i = 0 ; i< 10 ; i++){
            addToLinkedList(i);
        }
        System.out.println("Added Data");

        printLinkedList();
        reverseLinkedList();
        printLinkedList();


    }



    private static void reverseLinkedList() {//O(n) Time complexity, O(1) space complexity 

        //two temp pointers
        Node next = null, previous = null;

        while(head.next != null){
            next = head.next;//move next to next address
            head.next = previous;   //previous node will be the next node for head, so that head will point reverse         
            previous = head; //incrementing previous to the current node
            head = next; //incrementing head 

        }
        //at this point head points to last node and previous has the remaining reversed array
        head.next = previous;
        System.out.println("\nReversed");

    }


    /* Logic for adding and printing linked list*/
    private static void printLinkedList() {
        System.out.println("Printing linked list");
        Node temp = head;
        while(temp.next != null){
            System.out.print(temp.value+" ");
            temp = temp.next;
        }
        System.out.print(temp.value+" ");//print the value at the last node


    }

    private static void addToLinkedList(int value){
        if(head == null){
            head = new Node(value, null);
        }else{
            Node temp = head;
            while(temp.next != null){
                temp = temp.next;
            }
            temp.next = new  Node(value, null);
        }
    }

}

//Linked List definition 
class Node{
    int value;
    Node next;
    public Node(int value, Node next){
        this.value = value;
        this.next = next;
    }
}

Program's Output:

Added Data
Printing linked list
0 1 2 3 4 5 6 7 8 9 
Reversed
Printing linked list
9 8 7 6 5 4 3 2 1 0 

Hope it helps :)