class ListNode:
    def __init__(self, v):
        self.val = v
        self.next = None
    def __repr__(self):
      return "{} -> {}".format(self.val, self.next)

x = ListNode(1)
x.next = ListNode(3)
x.next.next = ListNode(5)

y = ListNode(2)
y.next = ListNode(4)
y.next.next = ListNode(6)

x  # returns 1 -> 3 -> 5 -> None
y  # returns 2 -> 4 -> 6 -> None