141. Linked List Cycle

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在 Q202 里用过的龟兔赛跑快慢指针，相遇即证明有环。

202. Happy Number

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        if not head: return False
        slow = head
        fast = head.next
        
        while fast != slow:
            if not fast or not fast.next: return False
            else:
                fast = fast.next.next
                slow = slow.next
                
        return True