000445-Add-Two-Numbers-II

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