链表反向并将新节点插入反向链表

Question

I'm learning DSA, and I was trying to implement the reverse operation of a linked list, but apparently the create method that I've written is not working after executing the reversal operation.

Here is the code:

import java.util.*;

public class reverse {
    Scanner sc = new Scanner(System.in);

    static class Node {
        int data;
        Node next;

        Node(int data) {
            this.data = data;
            this.next = null;
        }
    }

    Node head = null;
    Node tail = null;

    public void create() {
        System.out.println("Enter data");
        int value = sc.nextInt();
        Node newnode = new Node(value);
        if (head == null) {
            head = newnode;
            tail = newnode;
        } else {
            tail.next = newnode;
            tail = newnode;
        }
    }

    public void reversal() {
        Node prev = null;
        Node temp = head;
        Node nxt = null;
        while (temp != null) {
            nxt = temp.next;
            temp.next = prev;
            prev = temp;
            temp = nxt;
        }
        head = prev;
    }

    public void display() {
        Node temp = head;
        while (temp.next != null) {
            System.out.println(temp.data);
            temp = temp.next;
        }
        System.out.println(temp.data);
    }

    public static void main(String[] args) {
        int d = 0, a;
        Scanner sc = new Scanner(System.in);
        reverse obj = new reverse();
        do {
            System.out.println("1.Create\n2.Reverse\n3.Display");
            int choice = sc.nextInt();
            switch (choice) {
                case 1:
                    obj.create();
                    break;
                case 2:
                    obj.reversal2();
                    break;
                case 3:
                    obj.display();
                    break;
                default:
                    System.out.println("Enter a valid number!");
                    break;
            }
            System.out.println("1-Continue,0-Exit");
            a = sc.nextInt();
            if (a == 1) {
                d = 1;
            } else if (a != 1 && a != 0) {
                System.out.println("Enter a valid number");
            } else {
                System.out.println("Code Terminated!!");
                d = 0;
            }
        } while (d == 1 && d != 0);
    }
}

This is the output:

1.Create 
2.Reverse 
3.Display
1

Enter data
1

1-Continue,0-Exit
1

1.Create
2.Reverse
3.Display

1

Enter data
2

1-Continue,0-Exit
1

1.Create
2.Reverse
3.Display
1

Enter data
3

1-Continue,0-Exit
1

1.Create
2.Reverse
3.Display
2

1-Continue,0-Exit
1

1.Create
2.Reverse
3.Display
3

Numbers are:
3
2
1

1-Continue,0-Exit
1

After reversing the linked list.

1.Create
2.Reverse
3.Display
1

Enter data
4

1-Continue,0-Exit
1

1.Create
2.Reverse
3.Display
3

Numbers are:
3
4

1-Continue,0-Exit
0

I am not able to figure out what's wrong.

Answer 1

In your reversal method you should keep 3 temporary nodes:

• one to iterate the list (as you're doing).
• one maintaining the head of the reversed list.
• one to create a new node for the reversed list at each iteration of the list traversal.

During each loop, the new node for the reversed list should contain the info of the currently visited node, then you could attach to this node the reversed list (so the last node created points to the first nodes visited) and finally updated the head of the reversed list with the new node created.

Error Explanation

The reason why you couldn't add any further elements after reversing your list is because your tail node wasn't being updated. In fact, in your version tail was left referencing a non-existing node (the old list is gone once you update head with the new reversed list). In your while loop you need to set tail to the first new node created, as this will become the last node of your reversed list at the end of the loop, ie your tail.

public void reversal() {
    //Temp node to iterate the list
    Node temp = head;
    
    //Head of the reversed list
    Node headRev = null;
    
    //Temporary node to create at each iteration a new node for the reversed list
    Node newNode;

    //Resetting the tail
    tail = null;

    //Iterating the list
    while (temp != null) {
        
        //Creating a new node with the info of the current node
        newNode = new Node(temp.data);

        //Setting the tail of the new reversed list with the first node created
        tail = tail == null ? newNode : tail;
        
        //Appending to the new node the rest of the reversed list
        newNode.next = headRev;
        
        //Moving the head of the reversed list to the new node just created
        headRev = newNode;
        
        //Moving the iteration of the list to the next node
        temp = temp.next;
    }
    
    //Updating the head node of the list with the head of the reversed list
    head = headRev;
}