力扣：合并两个有序链表

该题目是合并2个有序的链表。主要实现思路如下：

1. 链表是有序的。比较的时候只要list1的值大于list2的中的就一直插入数据。反之亦然。
2. 要用链表才能通过，不能使用数组
3. 注意空链表以及终止条件

# Definition for singly-linked list.
class ListNode:
	def __init__(self, val=0, next=None):
	   	self.val = val
        self.next = next
class Solution:
    def appendList(self,list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        while list2:
            list1.next = ListNode(list2.val)
            list1 = list1.next
            list2 = list2.next

    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        if not list2:
            return list1
        if not list1:
            return list2
        if list1.val > list2.val:
            r = m = ListNode(list2.val)
            list2 = list2.next
            if not list2:
                self.appendList(m,list1)
                return r
        else:
            r = m = ListNode(list1.val)
            list1 = list1.next
        while list1:
            while list1.val > list2.val:
                m.next = ListNode(list2.val)
                m = m.next
                list2 = list2.next
                if not list2:
                    self.appendList(m,list1)
                    return r
            m.next = ListNode(list1.val)
            m = m.next
            list1 = list1.next
        self.appendList(m,list2)
        return r