Description

• You are given two linked lists representing two non-negative numbers. 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.
• Input: (2->4->3) + (5->6->4)
• Output: 7->0->8

很简单的链表问题，注意进位即可。

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

class Solution(object):
    def addTwoNumbers(self, l1, l2):
    """
    :type l1: ListNode
    :type l2: ListNode
    :rtype: ListNode
    """
    if not l1:
        return l2
    elif not l2:
        return l1
    
    carry = 0
    dummy_head = ListNode(0)
    current = dummy_head
    
    while l1 and l2:
        temp = l1.val + l2.val + carry
        l1 = l1.next
        l2 = l2.next
        carry = temp / 10
        current.next = ListNode(temp - 10*carry)
        current = current.next
    
    while l1:
        temp = l1.val + carry
        l1 = l1.next
        carry = temp / 10
        current.next = ListNode(temp - 10*carry)
        current = current.next
    
    while l2:
        temp = l2.val + carry
        l2 = l2.next
        carry = temp / 10
        current.next = ListNode(temp - 10*carry)
        current = current.next
    if carry == 1:
        current.next = ListNode(1)
    
    return dummy_head.next