检测环形链表

def solve():
    class ListNode:
        def __init__(____):
            self.val = val
            self.next = next
    
    # 创建有环链表
    head = ListNode(____)
    head.next = ListNode(____)
    head.next.next = ListNode(____)
    head.next.next.next = head.next  # 3指向2形成环
    
    slow = fast = head
    has_cycle = False
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow == fast:
            has_cycle = True
            break
    print(____)