21. Merge Two Sorted Lists

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.

Dummy node

/**
 * 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) {
    if(l1==null){
        return l2;
    }    
    if(l2==null){
        return l1;
    }
    ListNode dummy = new ListNode (0);
    ListNode head = dummy;
    
    while(l1!=null&&l2!=null){
    if(l1.val<l2.val){
        head.next=l1;
        l1=l1.next;
        head=head.next;
    }
    else{
        head.next=l2;
        l2=l2.next;
        head=head.next;
        }
    }
    
    while (l1!=null){
        head.next=l1;
        l1=l1.next;
        head=head.next;
    }
    while(l2!=null){
        head.next=l2;
        l2=l2.next;
        head=head.next;
    }
    return dummy.next;
    }
}