old_contents: # Time:  O(n)
  # Space: O(1)
  #
  # 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
  #
  # Definition for singly-linked list.
  class ListNode:
      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(0)
          current, 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
              carry, val = val / 10, val % 10
              current.next = ListNode(val)
              current = current.next
  
          if carry == 1:
              current.next = ListNode(1)
  
          return dummy.next
  
  if __name__ == '__main__':
      a, a.next, a.next.next = ListNode(2), ListNode(4), ListNode(3)
      b, b.next, b.next.next = ListNode(5), ListNode(6), ListNode(4)
      result = Solution().addTwoNumbers(a, b)
      print "{0} -> {1} -> {2}".format(result.val, result.next.val, result.next.next.val)
          
  
new_contents: # Time:  O(n)
  # Space: O(1)
  #
  # 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
  #
  # Definition for singly-linked list.
  class ListNode:
      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(0)
          current, 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
              carry, val = val / 10, val % 10 # val = val//10 if python 3 
              current.next = ListNode(val)
              current = current.next
  
          if carry == 1:
              current.next = ListNode(1)
  
          return dummy.next
  
  if __name__ == '__main__':
      a, a.next, a.next.next = ListNode(2), ListNode(4), ListNode(3)
      b, b.next, b.next.next = ListNode(5), ListNode(6), ListNode(4)
      result = Solution().addTwoNumbers(a, b)
      print "{0} -> {1} -> {2}".format(result.val, result.next.val, result.next.next.val)
          
  
subject: update difference on versions