Add Two Numbers Ii -

https://leetcode.com/problems/add-two-numbers-ii

Definition for singly-linked list. class ListNode: def init(self, val=0, next=None): self.val = val self.next = next

Algorithm

class Solution:
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        if not l1 and not l2:
            return None
        if not l1:
            return l2
        if not l2:
            return l1
        return reverse(addTwoNumbers(reverse(l1), reverse(l2)))

def reverse(l: ListNode) -> ListNode:
    prev = None
    head = None

    while l:
        head = l
        tmp = l.next
        l.next = prev
        prev = l
        l = tmp
    
    return head

def addTwoNumbers(l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
    head = current = ListNode()
    carry = 0
    while l1 or l2 or carry:
        val_1 = l1.val if l1 else 0
        val_2 = l2.val if l2 else 0
        raw_sum = val_1 + val_2 + carry
        sum = raw_sum % 10
        carry = raw_sum // 10
        current.next = ListNode(sum)
        current = current.next

        if l1:
            l1 = l1.next
        if l2:
            l2 = l2.next

    return head.next