有环链表

A:   1->5->10->11->18
           ↑       ↓
           ↑       ↓
           ↑ ← ← ← ←

A:   1->5->10->11->18->null

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param head: The first node of linked list.
     * @return: True if it has a cycle, or false
     */
    public boolean hasCycle(ListNode head) { 
        if (head == null || head.next == null) {
            return false;
        }
        
        ListNode slow = head, fast = head;
        
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                return true;
            }
        }
        return false;
    }
}