README.md

445. Add Two Numbers II

You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first 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.

Follow up:

What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7

Solutions (Python)

1. Stack

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
        stack1 = []
        stack2 = []

        while l1:
            stack1.append(l1.val)
            l1 = l1.next
        while l2:
            stack2.append(l2.val)
            l2 = l2.next

        head = None
        carry = 0

        while stack1 or stack2 or carry:
            digit = carry
            digit += stack1.pop() if stack1 else 0
            digit += stack2.pop() if stack2 else 0
            carry = 1 if digit > 9 else 0
            digit -= 10 if digit > 9 else 0

            temp = ListNode(digit)
            temp.next = head
            head = temp

        return head