### Problem

You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

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

Example 1:

Input: l1 = [7,2,4,3], l2 = [5,6,4] Output: [7,8,0,7]

Example 2:

Input: l1 = [2,4,3], l2 = [5,6,4] Output: [8,0,7]

Example 3:

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

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.

Follow up: Could you solve it without reversing the input lists?

### Solution

stack to keep track of the reverse val of the list

``````# 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]:
stack1, stack2 = [], []
while l1:
stack1.append(l1.val)
l1 = l1.next
while l2:
stack2.append(l2.val)
l2 = l2.next
res = None
carry = 0
while stack1 or stack2 or carry:
top1 = stack1.pop() if stack1 else 0
top2 = stack2.pop() if stack2 else 0

total = top1+top2+carry
curr = total%10
carry = total//10
newNode = ListNode(curr)
newNode.next = res
res = newNode

return res
``````

Last updated