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 contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example 1:

Bash

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)

Output: 7 -> 0 -> 8

Explanation: 342 + 465 = 807.

Example 2:

Bash

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

Solution Explanation

The approach for solving this problem will be, to traverse through each node of the linked list, carry out a sum on every step and return the final result. We just need to know the linked list data structure and the problem will be solved really easily.

solution.py

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = nex
class Solution:
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:

      dummy = ListNode()
      current = dummy
      carry = 0

      while (l1 or l2 or carry):
        n1 = l1.val if l1 else 0
        n2 = l2.val if l2 else 0
        s = n1 + n2 + carry

        current.next = ListNode(s%10)
        current = current.next
        carry = s//10

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