2-add-two-numbers.py

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        dummy = cur = ListNode()
        carry = 0
        while l1 or l2 or carry:
            if l1:
                carry += l1.val
                l1 = l1.next
            if l2:
                carry += l2.val
                l2 = l2.next
            carry, digit = divmod(carry, 10)
            cur.next = ListNode(digit)
            cur = cur.next
        return dummy.next
        
# time O(n)
# space O(n), creating a new linked list
# using linked list and sentinel node and two pointers