21. 合并两个有序链表

    // 递归
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }
        if (l1.val < l2.val) {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }
    // 迭代
    public ListNode mergeTwoLists2(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode prev = head;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                prev.next = l1;
                l1 = l1.next;
            } else {
                prev.next = l2;
                l2 = l2.next;
            }
            prev = prev.next;
        }
        prev.next = l1 == null ? l2 : l1;
        return head.next;
    }