问题描述：
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
方法1：
1 class Solution(object): 2 def mergeTwoLists(self, l1, l2): 3 """ 4 :type l1: ListNode 5 :type l2: ListNode 6 :rtype: ListNode 7 """ 8 p = dummy = ListNode(-1) 9 while l1 and l2: 10 if l1.val <= l2.val: 11 p.next = ListNode(l1.val) 12 l1 = l1.next 13 else: 14 p.next = ListNode(l2.val) 15 l2 = l2.next 16 p = p.next 17 p.next = l1 or l2 18 return dummy.next
方法2：递归
1 class Solution(object): 2 def mergeTwoLists(self, l1, l2): 3 """ 4 :type l1: ListNode 5 :type l2: ListNode 6 :rtype: ListNode 7 """ 8 if l1 == None: 9 return l2 10 if l2 == None: 11 return l1 12 mergel = None #设置新链表 13 if l1.val < l2.val: 14 mergel = l1 15 mergel.next = self.mergeTwoLists(l1.next, l2) 16 else: 17 mergel = l2 18 mergel.next = self.mergeTwoLists(l1, l2.next) 19 return mergel 20