141. 环形链表

快慢指针解法

func hasCycle(_ head: ListNode?) -> Bool {
        var slowTree = head
        var fastTree = head?.next

        while slowTree != nil || fastTree != nil {
            if slowTree === fastTree {
                return true
            }
            slowTree = slowTree?.next
            fastTree = fastTree?.next?.next
        }

        return false
    }

哈希表解法

func hasCycle(_ head: ListNode?) -> Bool {
        // 注意这里使用 Set, 使用 Array 会超时
        // ListNode 需要实现 Equatable, Hashable 协议
        var list: Set<ListNode> = []
        var head = head

        while head != nil {
            if list.contains(head!) {
                return true
            }
            list.insert(head!)
            head = head?.next
        }
        return false
    }

Swift 链表 及 实现协议

public class ListNode {
    public var val: Int
    public var next: ListNode?
    public init() { self.val = 0; self.next = nil }
    public init(_ val: Int) { self.val = val; self.next = nil }
    public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next }
}

extension ListNode: Equatable {
    public static func == (l: ListNode, r: ListNode) -> Bool {
        return l === r
    }
}

extension ListNode: Hashable {
    public func hash(into hasher: inout Hasher) {
        hasher.combine(ObjectIdentifier(self))
    }
}