[leetcode]Add Two Numbers @ Python

原题地址：https://oj.leetcode.com/problems/add-two-numbers/

题意：

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

解题思路：链表的基本操作。Two methods are provided. Method 1 is much slimmer

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

class Solution:
    # @return a ListNode
    def addTwoNumbers(self, l1, l2):
        # method 1: 14 Lines
        dummy, flag = ListNode(0), 0
        head = dummy
        while flag or l1 or l2:
            node = ListNode(flag)
            if l1:
                node.val += l1.val
                l1 = l1.next
            if l2:
                node.val += l2.val
                l2 = l2.next
            flag = node.val / 10
            node.val %= 10
            head.next = node  
            head = head.next  # head.next, head = node, node
        return dummy.next
        

        """
        # method 2: 24 Lines
        if not l1: return l2
        if not l2: return l1
        dummy = ListNode(0)
        p = dummy
        flag = 0
        while l1 and l2:
            tmp = l1.val + l2.val + flag
            p.next = ListNode( tmp % 10 )
            flag = tmp / 10
            l1, l2, p = l1.next, l2.next, p.next
        if l1:
            while l1:
                tmp = l1.val + flag
                p.next = ListNode( tmp % 10 )
                flag = tmp / 10
                l1, p = l1.next, p.next
        if l2:
            while l2:
                tmp = l2.val + flag
                p.next = ListNode( tmp % 10 )
                flag = tmp / 10
                l2, p = l2.next, p.next
        if flag == 1: p.next = ListNode(flag)
        return dummy.next
        """