翻转单链表

class Node:
    def __init__(self, value, p=None):
        self.data = value
        self.next = p

class Solution:
    def reverse_link(self, head):
        pre = None
        cur = head
        while cur:          #维护三个变量，循环地把当前节点next指向前一个节点
            tmp = cur.next
            cur.next = pre
            pre = cur
            cur = tmp
       return pre        # pre始终指向cur的前一个节点，当cur指向None的时候，pre刚好指向原链表的最后一个节点

if __name__ == '__main__':
    l = Node(1, Node(2, Node(3, Node(4))));    #构造一个链表1->2->3->4->None
    s = Solution()
    m = s.reverse_link(l)
    while m:
        print(m.data)
        m = m.next
    print(m)                     #输出链表 4->3->2->1->None