def sumLists(list1Node, list2Node, carry=0):
if not (list1Node or list2Node or carry > 0):
return None
sumNode = Node(carry)
list1Next = None
list2Next = None
# Add the listNode only if necessary, this handles varying lengths
if list1Node:
sumNode.value += list1Node.value
list1Next = list1Node.next
if list2Node:
sumNode.value += list2Node.value
list2Next = list2Node.next
carry = sumNode.value / 10
sumNode.value = sumNode.value % 10
# hands off carry and next nodes of both lists recursively
nextSumNode = sumLists(list1Next, list2Next, carry)
# After node is received, take current node and put in front
sumNode.next = nextSumNode
return sumNode
