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Java Program To Merge A Linked List Into Another Linked List At Alternate Positions

  • Difficulty Level : Easy
  • Last Updated : 31 Mar, 2022

Given two linked lists, insert nodes of the second list into the first list at alternate positions of the first list. 
For example, if first list is 5->7->17->13->11 and second is 12->10->2->4->6, the first list should become 5->12->7->10->17->2->13->4->11->6 and second list should become empty. The nodes of the second list should only be inserted when there are positions available. For example, if the first list is 1->2->3 and the second list is 4->5->6->7->8, then the first list should become 1->4->2->5->3->6 and the second list to 7->8.
Use of extra space is not allowed (Not allowed to create additional nodes), i.e., insertion must be done in-place. The expected time complexity is O(n) where n is the number of nodes in the first list.

The idea is to run a loop while there are available positions in first loop and insert nodes of second list by changing pointers. Following are implementations of this approach. 

Java




// Java program to merge a linked list
// into another at alternate positions
class LinkedList
{
    // head of list
    Node head; 
 
    // Linked list Node
    class Node
    {
        int data;
        Node next;
        Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    /* Inserts a new Node at front
       of the list. */
    void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);
 
        // 3. Make next of new Node as head
        new_node.next = head;
 
        /* 4. Move the head to point to new
              Node */
        head = new_node;
    }
 
    // Main function that inserts nodes of
    // linked list q into p at alternate positions.
    // Since head of first list never changes and
    // head of second list/ may change, we need single
    // pointer for first list and double pointer for
    // second list.
    void merge(LinkedList q)
    {
        Node p_curr = head,
             q_curr = q.head;
        Node p_next, q_next;
 
        // While there are available positions
        // in p;
        while (p_curr != null &&
               q_curr != null)
        {
            // Save next pointers
            p_next = p_curr.next;
            q_next = q_curr.next;
 
            // make q_curr as next of p_curr
            // change next pointer of q_curr
            q_curr.next = p_next;
 
            // change next pointer of p_curr
            p_curr.next = q_curr;
 
            // update current pointers for
            // next iteration
            p_curr = p_next;
            q_curr = q_next;
        }
        q.head = q_curr;
    }
 
    // Function to print linked list
    void printList()
    {
        Node temp = head;
        while (temp != null)
        {
           System.out.print(temp.data + " ");
           temp = temp.next;
        }
        System.out.println();
    }
 
    // Driver code
    public static void main(String args[])
    {
        LinkedList llist1 =
        new LinkedList();
        LinkedList llist2 =
        new LinkedList();
        llist1.push(3);
        llist1.push(2);
        llist1.push(1);
 
        System.out.println(
               "First Linked List:");
        llist1.printList();
 
        llist2.push(8);
        llist2.push(7);
        llist2.push(6);
        llist2.push(5);
        llist2.push(4);
 
        System.out.println(
               "Second Linked List:");
 
        llist1.merge(llist2);
 
        System.out.println(
               "Modified first linked list:");
        llist1.printList();
 
        System.out.println(
               "Modified second linked list:");
        llist2.printList();
    }
}
// This code is contributed by Rajat Mishra

Output: 

First Linked List:
1 2 3
Second Linked List:
4 5 6 7 8
Modified First Linked List:
1 4 2 5 3 6
Modified Second Linked List:
7 8 

Time Complexity: O(N)

Auxiliary Space: O(1)

Please refer complete article on Merge linked list into another linked list at alternate positions for more details!


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