Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4 
Output: 1->1->2->3->4->4

Problem Clarification

We cannot copy the nodes.

Idea – 1

Merge routine in mergesort, time and space .

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(-1);
        ListNode t = dummy;
        
        while(l1 != null && l2 != null)
        {
            if(l1.val < l2.val)
            {
                t.next = l1;
                l1 = l1.next;
            }
            else
            {
                t.next = l2;
                l2 = l2.next;
            }
            
            t = t.next;
        }
        
        if(l1 != null)
        {
            t.next = l1;
        }
        else
        {
            t.next = l2;
        }
        
        return dummy.next;
    }
}


Runtime: 1 ms, faster than 88.84% of Java online submissions for Merge Two Sorted Lists.
Memory Usage: 37.1 MB, less than 97.90% of Java online submissions for Merge Two Sorted Lists.