# 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
        """
        
        dummy = ListNode('-inf')
        cur, carry = dummy, 0
        
        while l1 or l2:
            val = carry
            if l1:
                val += l1.val
                l1 = l1.next
            if l2:
                val += l2.val
                l2 = l2.next
            val, carry = val % 10, val / 10
            cur.next = ListNode(val)
            cur = cur.next
            
        if carry == 1:
            cur.next = ListNode(1)
            
        return dummy.next
        
  @@@@follow up
  # 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
        """
        stk1, stk2 = [], []
        while l1:
            stk1.append(l1.val)
            l1 = l1.next
        while l2:
            stk2.append(l2.val)
            l2 = l2.next
        
        sum = 0
        pre, head = None, None
        while stk1 or stk2:
            sum /= 10
            if stk1:
                sum += stk1.pop()
            if stk2:
                sum += stk2.pop()
                
            head = ListNode(sum % 10)
            head.next = pre
            pre = head
        if sum >= 10:
            head = ListNode(sum / 10)
            head.next = pre
        return head
            
