环形链表

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def solve():
    def hasCycle(head):
        if not head or not head.next:
            return False
        slow = head
        fast = head.next
        while slow != fast:
            if not fast or not fast.next:
                return False
            slow = slow.next
            fast = fast.next.next
        return True
    head = ListNode(3, ListNode(2, ListNode(0, ListNode(-4))))
    print(hasCycle(head))