求链表中间值

一个快指针，一个慢指针

# 先定义一个链表
class Node:
    def __init__(self,data,next):
        self.data = data
        self.next = next
n1 = Node('n1',None)    # 尾部节点tail
n2 = Node('n2',n1)
n3 = Node('n3',n2)
n4 = Node('n4',n3)
n5 = Node('n5',n4) # 初始节点head

p1 = n5
p2 = n5
while p2.next is not None and p2.next.next is not None:
    p1 = p1.next
    p2 = p2.next.next
print (p1.data)