使用Floyd算法判断单链表中是否存在环（链表头节点为head）的逻辑为：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None。请补全以下代码横线处的内容：

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