2. Add Two Numbers

[mengyushi]

# 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:

        dummy = ListNode(None)
        pre = dummy
        carry = 0

        while l1 and l2:
            val = l1.val + l2.val + carry
            if val >= 10:
                val-=10
                carry = 1
            else:
                carry = 0

            pre.next = ListNode(val)
            pre = pre.next
            l1, l2 = l1.next, l2.next

        l = l1 or l2

        while l:
            val = l.val + carry
            if val >= 10:
                val-=10
                carry = 1
            else:
                carry = 0 

            pre.next = ListNode(val)
            pre = pre.next
            l = l.next

        if carry:
            pre.next = ListNode(1)

        return dummy.next