题解 | #判断链表中是否有环#

/**
 * public class ListNode {
 *   public var val: Int
 *   public var next: ListNode?
 *   public init(_ val: Int = 0, _ next: ListNode? = nil) {
 *     self.val = val
 *     self.next = next
 *   }
 * }
 */
public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @return bool布尔型
     */
    func hasCycle ( _ head: ListNode?) -> Bool {
        // write code here
        var first = head
        var second = head
        while first?.next != nil && second?.next?.next != nil {
            first = first?.next
            second = second?.next?.next
            if first!.val == second!.val {
                return true
            }
        }
        return false
    }
}