## 題解

## 快慢指針

跑操場倒追原理

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        slow,fast = head,head
        while slow and fast:
            fast = fast.next
            slow = slow.next
            if fast:
                fast = fast.next
                if fast == slow:
                    return True
        return False