2.Add Two Numbers Leetcode Python New season for 2016

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

# class ListNode(object):#     def __init__(self, x):#         self.val = x#         self.next = Noneclass Solution(object):    def addTwoNumbers(self, l1, l2):        """        :type l1: ListNode        :type l2: ListNode        :rtype: ListNode        """        value_sum = 0        dummy_head = head = ListNode(None)        while l1 or l2:            if l1:                value_sum += l1.val                l1 = l1.next            if l2:                value_sum += l2.val                l2 = l2.next            head.next = ListNode(value_sum % 10)            head = head.next            value_sum /= 10        if value_sum == 1:            head.next = ListNode(1)        return dummy_head.next

traverse the whole tree and sum up the value in the node by order.

O(n) time and O(n) space