class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode l1 = list1, l2 = list2;
        ListNode dummy = new ListNode(-1), d = dummy;

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

        return dummy.next;
    }
}

class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if(list1 == null) return list2;
        if(list2 == null) return list1;

        if(list1.val < list2.val){
            list1.next = mergeTwoLists(list1.next, list2);
            return list1;
        }else{
            list2.next = mergeTwoLists(list1, list2.next);
            return list2;
        }
    }
}