/**
 * 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 cur = dummy;

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

        if (l1 != null) cur.next = l1;
        if (l2 != null) cur.next = l2;

        return dummy.next;    
    }
}