2. Add Two Numbers

Explain

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order 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.

Example

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)

Output: 7 -> 0 -> 8

Explanation: 342 + 465 = 807.

Solve

# 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:
        head, curr = ListNode(0)
        carry = 0
        while l1 or l2 or carry:
            if l1:
                carry += l1.val
                l1 = l1.next
            if l2:
                carry += l2.val
                l2 = l2.next
            curr.next = ListNode(carry % 10)
            curr = curr.next
            carry //= 10
        return head.next

Performance

Runtime: 88 ms, faster than 16.90% of Python3 online submissions for Add Two Numbers.

Memory Usage: 14 MB, less than 5.67% of Python3 online submissions for Add Two Numbers.