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
递归和迭代：

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        ListNode head = new ListNode(0);
        if(l1.val < l2.val){
            head = l1;
            head.next = mergeTwoLists(l1.next, l2);
        }else{
            head = l2;
            head.next = mergeTwoLists(l2.next, l1);
        }
        return head;
    }

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        ListNode dummy = new ListNode(0);
        ListNode node = dummy;
        while(l1 != null && l2 != null){
            if(l1.val < l2.val){
                node.next = l1;
                node = node.next;
                l1 = l1.next;
            }else{
                node.next = l2;
                node = node.next;
                l2 = l2.next;
            }
        }
        if(l1 != null){
            node.next = l1;
        }else{
            node.next = l2;
        }
        return dummy.next;
    }