2.py

# Name: Add Two Numbers
# Link: https://leetcode.com/problems/add-two-numbers/
# Method: Build new linked list, keep going as long as one head present (or a carry)
# Time: O(n)
# Space: O(n)
# Difficulty: Medium


class ListNode:
 def __init__(self, val=0, next=None):
 self.val = val
 self.next = next


class Solution:
 def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
 # Consider case when not same len
 res_head = ListNode()
 res_now = res_head

 l1_head = l1
 l2_head = l2
 carry = 0
 while l1_head or l2_head or carry:
 sum_now = (
 (l1_head.val if l1_head else 0)
 + (l2_head.val if l2_head else 0)
 + carry
 )
 res_now.next = ListNode(sum_now % 10)

 carry = sum_now // 10
 res_now = res_now.next
 l1_head = l1_head.next if l1_head else None
 l2_head = l2_head.next if l2_head else None

 return res_head.next