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

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``public` `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