1.開辟字典，有額外空間

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        dict1={}
        if not head or not head.next:
            return False
        count=-1
        while head and head not in dict1:
            count+=1
            dict1[head]=count
            if head.next:
                head = head.next
            else:
                return False
        return True

2.快慢指針，沒有額外空間

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        if not head or not head.next:
            return False
        quick=slow=head
        # 下邊這個循環(huán)不用擔(dān)心死循環(huán)，因為只要出現(xiàn)環(huán)，就會被if語句捕捉，不會永遠轉(zhuǎn)下去的
        # 若無環(huán)，當快指針遍歷到最后時，就會結(jié)束循環(huán)
        while quick.next and quick.next.next:
            quick=quick.next.next
            slow=slow.next
            if quick==slow:
                return True
        return False