下列python代码用Floyd判断一个单链表中是否存在环，链表的头节点为head，即用两个指针在链表上前进：slow 每次走 1 步，fast 每次走 2 步，若存在环，fast 终会追上 slow（相遇）；若无环，fast 会先到达 nullptr。横线上应填写（ ）。

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

def hasCycle(head: ListNode) -> bool:
    if not head or not head.next:
        return False
    slow = head
    fast = head.next
    while fast and fast.next:
        if slow == fast:
            return True
        _________________
    return False