LeetCode，给定一个链表，判断链表中是否有环

func hasCycle(head *ListNode) bool {
    maps := map[*ListNode]struct{}{}
    for head != nil {
        if _,ok := maps[head.Next]; ok {
            return true
        }else{
            maps[head.Next] = struct{}{}
        }
        head = head.Next
    }
    return false
}

func hasCycle(head *ListNode) bool {
    if head == nil || head.Next == nil {
        return false
    }
    slow := head
    fast := head.Next
    for fast != slow {
        if fast == nil || fast.Next == nil {
            return false
        }
        slow = slow.Next
        fast = fast.Next.Next
    }
    return true
}