Merge Two Sorted Linked Lists

You are given two sorted linked lists. You have to merge them to produce a combined sorted linked list. You need to return the head of the final linked list.

Note:

The given linked lists may or may not be null.

For example:

If the first list is: 1 -> 4 -> 5 -> NULL and the second list is: 2 -> 3 -> 5 -> NULL

The final list would be: 1 -> 2 -> 3 -> 4 -> 5 -> 5 -> NULL

Input Format:

The first line of input contains the elements of the first linked list separated by a single space and terminated by -1. Hence, -1 would never be a list element.

The second line of input contains the elements of the second linked list separated by a single space and terminated by -1.

Output Format:

Print the final linked list. The elements of the linked list must be separated by a single space and terminated by -1.

Note:

You are not required to print the expected output, it has already been taken care of. Just implement the function.

Constraints:

1 <= L <= 10^5
1 ≤ data ≤ 10^6 and data != -1

Where L is the number of nodes in either of the two Linked List.

Time Limit: 1 sec 

Follow-up:

Try to solve this problem in linear time complexity and constant space complexity.