':type l1: ListNode
:type l2: ListNode
:rtype: ListNode'

def addTwoNumbers(self, l1, l2):

    (stk1, stk2) = ([], [])
    while l1:
        stk1.append(l1.val)
        l1 = l1.next
    while l2:
        stk2.append(l2.val)
        l2 = l2.next
    (prev, head) = (None, None)
    sum = 0
    while (stk1 or stk2):
        sum /= 10
        if stk1:
            sum += stk1.pop()
        if stk2:
            sum += stk2.pop()
        head = ListNode((sum % 10))
        head.next = prev
        prev = head
    if (sum >= 10):
        head = ListNode((sum / 10))
        head.next = prev
    return head
