LeetCode某題：用兩個連結串列分別表示兩個正整數，低位在前，高位在後，231表示1->3->2，99表示9->9，會保證末尾不為0，連結串列長度不固定，然後求和，和也是用相同形式的連結串列表示。

該題為例，

class ListNode():
    def __init__(self,x):
        self.val=x
        self.next=None
class SingleLinkList:
    def __init__(self, node=None):
        self.__head = node

    def is_Empty(self):
        
 
return self.__head is None

    def append(self, item): # 尾插法
        node = ListNode(item)
        if self.is_Empty():
            self.__head = node
        else:
            cur = self.__head
            while cur.next is not None: 
                cur = cur.next
            cur.next = node

    
 
def find_head(self):
        return self.__head
class Solution():
    def listsum(self,p,q):
        flag=0
        temp=ListNode(-1)
        res=temp
        while p or q or flag:
            p_val=p.val if p else 0
            q_val=q.val if q else 0
            temp.next=ListNode((p_val+q_val+flag)%10)
            flag
 
=(p_val+q_val+flag)//10
            if p:p=p.next
            if q:q=q.next
            temp=temp.next
        return res.next              
mm = Solution()
head = ListNode(None)
cur = head
for i in [1,2]:
    cur.next = ListNode(i)
    cur = cur.next
head = head.next
head2 = ListNode(None)
cur2 = head2
for i in [9,9]:
    cur2.next = ListNode(i)
    cur2 = cur2.next
head2 = head2.next
res = mm.listsum(head,head2)
while res:
    print(res.val, end=' ')
    res = res.next