2. Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list

Example 1:

• Input: l1 = [2,4,3], l2 = [5,6,4]
• Output: [7,0,8]
• Explanation: 342 + 465 = 807.

Example 2:

• Input: l1 = [0], l2 = [0]
• Output: [0]

Example 3:

• Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
• Output: [8,9,9,9,0,0,0,1]

Constraints:

• The number of nodes in each linked list is in the range [1, 100].
• 0 <= Node.val <= 9
• It is guaranteed that the list represents a number that does not have leading zeros.

Approach

Solution

# 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 = ListNode()
        carry = 0
        curr = dummy
        while l1 or l2 or carry:
            s = (l1.val if l1 else 0) + (l2.val if l2 else 0) + carry
            carry, val = divmod(s, 10)

            curr.next = ListNode(val)
            curr = curr.next

            l1 = l1.next if l1 else None
            l2 = l2.next if l2 else None

        return dummy.next

Explanation