判断链表中是否有环

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

#
# 
# @param head ListNode类 
# @return bool布尔型
#
class Solution:
    def hasCycle(self , head ):
        # write code here
        if not head:
            return False
        slow, fast = head, head
        while 1:
            if fast and fast.next: #没有环的情况下，保证fast最后停在none
                slow = slow.next
                fast = fast.next.next
                if fast == slow:
                    return True
            else:
                return False