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445. Add Two Numbers II

Difficulty Topics

Description

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

add-two-numbers-ii.py
# 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]:
        s1, s2 = [], []
        curr = res = ListNode(-1)

        while l1:
            s1.append(l1.val)
            l1 = l1.next

        while l2:
            s2.append(l2.val)
            l2 = l2.next

        carry = 0

        while s1 or s2 or carry:
            if s1:
                carry += s1.pop()

            if s2:
                carry += s2.pop()

            curr.next = ListNode(carry % 10)
            curr = curr.next

            carry //= 10

        def reverseLL(head):
            prev = None

            while head:
                nxt = head.next
                head.next = prev
                prev = head
                head = nxt

            return prev

        return reverseLL(res.next)