21. Merge Two Sorted Lists

思路：比较两个列表首节点，按照大小合成一个新链表，如果一个链表为空，则把另外的链表直接合入新链表。（递归）

public ListNode mergeTwoList(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }

        ListNode mergeHead;
        if (l1.val < l2.val) {
            mergeHead = l1;
            mergeHead.next = mergeTwoList(l1.next,l2);
        }
        else {
            mergeHead = l2;
            mergeHead.next = mergeTwoList(l1, l2.next);
        }
        return mergeHead;
    }