小练习：lintcode466. 链表节点计数

题目：

样例：

"""
Definition of ListNode
class ListNode(object):

def __init__(self, val, next=None):
self.val = val
self.next = next
"""

class Solution:
"""
@param: head: the first node of linked list.
@return: An integer
"""
def countNodes(self, head):
# write your code here
i = 0
p = head
while(p):
i += 1
p = p.next
return i