LeetCode--021--合并两个有序链表

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：

 class Solution(object):
     def mergeTwoLists(self, l1, l2):
         """
         :type l1: ListNode
         :type l2: ListNode
         :rtype: ListNode
         """
         p = dummy = ListNode(-1)
         while l1 and l2:
             if l1.val <= l2.val:
                 p.next = ListNode(l1.val)
                 l1 = l1.next
             else:
                 p.next = ListNode(l2.val)
                 l2 = l2.next
             p = p.next
         p.next = l1 or l2
         return dummy.next

方法2：递归

 class Solution(object):
     def mergeTwoLists(self, l1, l2):
         """
         :type l1: ListNode
         :type l2: ListNode
         :rtype: ListNode
         """
         if l1 == None:
             return l2
         if l2 == None:
             return l1
         mergel = None #设置新链表
         if l1.val < l2.val:
             mergel = l1
             mergel.next = self.mergeTwoLists(l1.next, l2)
         else:
             mergel = l2
             mergel.next = self.mergeTwoLists(l1, l2.next)
         return mergel