将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。
输入：l1 = [1,2,4], l2 = [1,3,4]输出：[1,1,2,3,4,4]
示例 2：
输入：l1 = [], l2 = []输出：[]
示例 3：
输入：l1 = [], l2 = [0]输出：[0]
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; } }}
class Solution { public ListNode mergeTwoLists(ListNode l1, ListNode l2) { ListNode dummyHead = new ListNode(0); ListNode tail = dummyHead; while (l1 != null && l2 != null) { if (l1.val < l2.val) { tail.next = l1; l1 = l1.next; } else { tail.next = l2; l2 = l2.next; } tail = tail.next; } tail.next = l1 == null ? l2 : l1; return dummyHead.next; }}