Leetcode 21.合并两个有序链表

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        l1,l2：链表结点
        """
        head=ListNode(-1)      #构造辅助链表
        prev=head              #辅助链表指针
        while l1 and l2:
            if l1.val>l2.val:
                prev.next=l2
                l2=l2.next
            else:
                prev.next=l1
                l1=l1.next
            prev=prev.next
        prev.next=l1 if l1 is not None else l2
        return head.next       #返回结果链表头结点