Lcof 25.合并两个排序的链表

输入两个递增排序的链表，合并这两个链表并使新链表中的节点仍然是递增排序的。

示例1：

​ 输入：1->2->4, 1->3->4
​ 输出：1->1->2->3->4->4

迭代

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(-1);
        ListNode res = head;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                res.next = l1;
                l1 = l1.next;
            } else {
                res.next = l2;
                l2 = l2.next;
            }
            res = res.next;
        }
        if (l1 == null) {
            res.next = l2;
        }
        if (l2 == null) {
            res.next = l1;
        }
        return head.next;
    }
}

递归

class Solution {
    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;
        }
    }
}