Reverse a singly-linked list.

reverse.py

class List:
    def __init__(self, data, next):
        self.data = data
        self.next = next
    def __repr__(self):
        return 'List({0.data!r}, {0.next!r})'.format(self)
    def str_helper(self):
        if self.next == None:
            return '{0.data!s}'.format(self)
        else:
            return '{0.data!s}, {1}'.format(self, self.next.str_helper())
    def __str__(self):
        return '({0!s})'.format(self.str_helper())

def reverse(head):
    prev = None
    while head != None:
        save_next = head.next
        head.next = prev
        prev = head
        head = save_next
    return prev

def reverse2(head):
    prev = None
    while head != None:
        save_head = head
        head = head.next
        save_head.next = prev
        prev = save_head
    return prev