2. Add Two Numbers.py

"""
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.
"""
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
# 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: ListNode, l2: ListNode) -> ListNode:
        prev = ListNode(-100)
        cur = prev
        total,carry = 0,0
        while l1 or l2:
            x =  l1.val if l1 else 0
            y = l2.val if l2 else 0
            total = (x + y + carry) % 10
            carry = (x + y + carry) // 10
            cur.next = ListNode(total)
            cur = cur.next
            if l1:
                l1 = l1.next
            if l2:
                l2 = l2.next
        if carry:
            cur.next = ListNode(carry)
        return prev.next