https://leetcode.com/problems/merge-two-sorted-lists/description/#

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode MergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode lastNode = dummy;

        while (l1 != null && l2 != null)
        {
            if (l1.val < l2.val)
            {
                lastNode.next = l1;
                l1 = l1.next;
            }
            else
            {
                lastNode.next = l2;
                l2 = l2.next;
            }
            lastNode = lastNode.next;
        }

        if (l1 != null)
        {
            lastNode.next = l1;
        }
        else
        {
            lastNode.next = l2;
        }

        return dummy.next;

    }
}