将两个有序链表合并为一个新的有序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 

示例：

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

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode(int x) { val = x; }* }*/// 方法一：递归方式public static ListNode mergeTwoLists1(ListNode l1, ListNode l2) {ListNode result = null;if (l1 == null) {return l2;}if (l2 == null) {return l1;}if (l1.val <= l2.val) {result = l1;result.next = mergeTwoLists1(l1.next, l2); // 递归} else {result = l2;result.next = mergeTwoLists1(l1, l2.next);}return result;}// 方法二：非递归方式public static ListNode mergeTwoLists2(ListNode l1, ListNode l2) {if (l1 == null) {return l2;}if (l2 == null) {return l1;}ListNode mergeNode = null;if (l1.val <= l2.val) {mergeNode = l1;l1 = l1.next;} else {mergeNode = l2;l2 = l2.next;}ListNode temp = mergeNode;while (l1 != null && l2 != null) {if (l1.val <= l2.val) {temp.next = l1;l1 = l1.next;} else {temp.next = l2;l2 = l2.next;}temp = temp.next;}if (l1 != null) {temp.next = l1;}if (l2 != null) {temp.next = l2;}return mergeNode;}