class ListNode:

    def __init__(self, x):

        self.val = x

        self.next = None

    def __repr__(self):# 显示方式

        return "{}->{}".format(self.val,self.next)

class Solution:

    def mergeTwoLists(self, l1, l2):

        """

        :type l1: ListNode

        :type l2: ListNode

        :rtype: ListNode

        """

        if not l1:return l2

        if not l2:return l1

        current =tmp= ListNode(0) #current 和 tmp 指向同一个头结点

        while l1 and l2:

            if l1.val < l2.val:

                current.next = l1

                l1=l1.next

            else:

                current.next = l2

                l2=l2.next

            current = current.next #单向链表，指向最后一个结点"指针"

        current.next = l1 or l2 #current链表最后一个结点的"指针"指向l1或者l2的头结点

        return tmp.next  #current的指针指向了l1或者l2的头结点，返回tmp完整链表