两数相加

思路：因为数是用链表逆序表示的，因此先用列表将这些数提取出来。然后每一位对应相加大于10就进位，进位用carry表示。有两个特殊情况要考虑：（1）象5+5这种，两个数的位数一样，最后要增加一位。（2）象1+99这种，两个数的位数不一样，先将1+9相加跳出循环，后面是carry+9=10，又出现了一个进位。

# 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
        """
        stack1 = []
        stack2 = []
        while l1:
            stack1.append(l1.val)
            l1 = l1.next
        while l2:
            stack2.append(l2.val)
            l2 = l2.next
        if len(stack1) > len(stack2):
            stack1, stack2 = stack2, stack1
        carry = 0
        head = ListNode(-1)
        p = head
        while stack1:
            tmp = stack1.pop(0) + stack2.pop(0) + carry
            carry = tmp // 10
            node = ListNode(tmp%10)
            p.next = node
            p = p.next
        if not stack2 and carry == 1:
            node = ListNode(1)
            p.next = node 
            p = p.next
            carry = 0
        elif stack2:
            while stack2:
                tmp = stack2.pop(0) + carry
                carry = tmp // 10
                node = ListNode(tmp%10)
                p.next = node
                p = p.next
            if carry == 1:
                node = ListNode(1)
                p.next = node 
                p = p.next
        return head.next