2. add-two-numbers

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
        ans=ListNode(0) #dummy head
        tmp=ans
        left=0
        while l1 or l2:
            x=l1.val if l1 !=None else 0
            y=l2.val if l2 !=None else 0
            sum=x+y+left
            node=ListNode(sum%10)
            left=sum//10

            tmp.next=node
            tmp=tmp.next
            l1=l1.next if l1 !=None else l1
            l2=l2.next if l2 !=None else l2
        if left>0:
            tmp.next=ListNode(left)
            
        return ans.next