在 java 中将备用节点从一个单链表复制到另一个链表

Question

I'm trying to write a method called copyAlternate which copy alternate elements from a single linked list and put them in another Single linked list sent as a parameter in the method

for example: if the first Single Linked List is( 4,6,10,12,2) the method should generate a single linked list with (4,10,2)

here is my code:

public boolean copyAlternate(SingleLinkedList<E> list1)
{

        if(head==null)
            return false;

        Node <E> temp = head;
        ArrayList <E> a1 = new ArrayList<E>();

        while(temp!=null) {
            a1.add(temp.data);
            temp=temp.next;
        }

        Node<E> tmp1=list1.head;
        for(int i=0;i<a1.size();i=i+2){

            if(list1.head==null) {
                list1.head =new Node(a1.get(i));
                tmp1=head;
                size++;
            }
            else
            {

             tmp1.next=new Node(a1.get(i));
             size++;
             tmp1=tmp1.next;
            }
        }
        return true;
    }

I got only 4 from method not (4,10,2) so what is the problem with my code?

Answer 1

Here is a full example if you need it, try not to copy it entirely but study it and understand what it's doing:

public class Main {

    public static void main(String[] args) {
        SingleLinkedList<Integer> list = new SingleLinkedList<>();
        // (4, 6, 10, 12, 2)
        list.add(4).add(6).add(10).add(12).add(2);
        SingleLinkedList<Integer> newList = copyAlternate(list);
        System.out.println(newList);
        // prints: (14, 10, 2)
    }

    public static class Node<T> {
        public Node<T> next;
        public T data;

        public Node() {}
        public Node(T data) {this.data = data;}
    }

    public static class SingleLinkedList<T> {
        private Node<T> head;
        private Node<T> tail;

        public SingleLinkedList<T> add(T data) {
            if (tail != null) {
                tail.next = new Node<>(data);
                tail = tail.next;
            } else {
                head = new Node<>(data);
                tail = head;
            }
            return this;
        }

        @Override public String toString() {
            StringBuilder sb = new StringBuilder();
            if (head != null) {
                sb.append(head.data);
                Node<T> curr = head.next;
                while (curr != null) {
                    sb.append(", ").append(curr.data);
                    curr = curr.next;
                }
            }
            return sb.toString();
        }
    }

    /**
     * Copies only the even index nodes (0, 2, 4..) into a new list and returns that list. 
     * Doesn't create a deep copy of data
     */
    public static <T> SingleLinkedList<T> copyAlternate(SingleLinkedList<T> list) {
        Objects.requireNonNull(list, "list");
        SingleLinkedList<T> newList = new SingleLinkedList<>();
        Node<T> other = list.head;
        if (other != null) {
            // copy other data to head of new list and remember that as the current node
            Node<T> curr = newList.head = new Node<>(other.data);
            // get the next next other node
            other = other.next;
            if (other != null) {
                other = other.next;
            }
            while (other != null) {
                // copy the data into the next node then remember that as the current node
                curr = curr.next = new Node<>(other.data);
                // get the next next other node
                other = other.next;
                if (other != null) {
                    other = other.next;
                }
            }
        }
        return newList;
    }
}

Answer 2

Another way you can achieve it. Hope it will help.

// define the structure of a single node
class ListNode<T>
{
   public T data;

   public ListNode<T> next;

   public ListNode(T data)
   {
      this.data = data;
      this.next = null;
   }
}

public class SingleLinkedListGenerics<T>
{
   private ListNode<T> head = null;

   private ListNode<T> tail = null;

   // used to insert a node at the end of linked list
   public void insertLast(T data)
   {
      ListNode<T> newNode = new ListNode<>(data);

      if (head == null)
      {
         head = tail = newNode;
      }
      else
      {
         tail.next = newNode;
         tail = newNode;
      }
   }

   // For printing Linked List
   public void displayList()
   {
      System.out.println("\nPrinting LinkedList (head --> last) ");
      ListNode<T> current = head;
      while (current != null)
      {
         System.out.println(current.data + ", ");
         current = current.next;
      }
   }

   public static <T> SingleLinkedListGenerics<T> copyAlternate(SingleLinkedListGenerics<T> list)
   {

      if (list.head == null)
         return null;

      SingleLinkedListGenerics<T> newList = new SingleLinkedListGenerics<>();

      ListNode<T> tmp = list.head;

      ListNode<T> newNode = new ListNode<>(tmp.data); // getting the first node
      newList.head = newNode;
      newList.tail = newNode;

      tmp = tmp.next;

      while (tmp != null && tmp.next != null) //
      {
         tmp = tmp.next; // skipping one node

         newList.insertLast(tmp.data);

         tmp = tmp.next;
      }

      return newList;
   }


   public static void main(String args[])
   {
      SingleLinkedListGenerics<Integer> myLinkedlist = new SingleLinkedListGenerics<>();
      myLinkedlist.insertLast(4);
      myLinkedlist.insertLast(6);
      myLinkedlist.insertLast(10);
      myLinkedlist.insertLast(12);
      myLinkedlist.insertLast(2);

      myLinkedlist.displayList();

      SingleLinkedListGenerics<Integer> myLinkedlist1 = copyAlternate(myLinkedlist);

      myLinkedlist1.displayList();
   }
}