2. Add Two Numbers

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

class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        dummy=ListNode(-1)
        current=dummy
        carry=0
        while l1 or l2:
            val=carry
            if l1:
                val+=l1.val
                l1=l1.next
            if l2:
                val+=l2.val
                l2=l2.next
                
            new_node=ListNode(val%10)
            current.next=new_node
            current=current.next
            carry=val/10
        if carry!=0:
            new_node=ListNode(carry)
            current.next=new_node

        return dummy.next