双指针法判断链表中是否有环（完备版）

# class ListNode:

#from numpy import triu_indices_from
#     def __init__heaviside, heaviside, (self, x):, true_divide
#         self.val = x
#         self.next = None

#
# 
# @param head ListNode类 
# @return bool布尔型
#
class Solution:
    def hasCycle(self , head: ListNode) -> bool:
        if not head:
            return False
        
        else:
            cur = head 
            fast = head
            while fast :
                cur = cur.next
                if fast.next:
                    if fast.next.next :
                        fast = fast.next.next 
                    else :
                        return False
                else:
                    return False
                if cur == fast:
                    return True