判断一个链表是否有环--剑指offer

class ListNode {
String val;
ListNode next;

public ListNode(String val) {
this.val = val;
}
}

public class Main {
boolean hasCycle(ListNode head) {
ListNode fast = head, slow = head;
while (fast != null && fast.next != null) {//这个条件“fast.next != null”需要加上，防止一个结点时报错。
fast = fast.next.next;//如果是3->null不报错，如果是如果是3->null->null则报错，因为空结点没有下一次个结点
//            fast = fast.next.next.next.next;这里一次往后走四步也行，三步也行
slow = slow.next;
if (fast == slow)
return true;
}
return false;
}//hasCycle

//测试
public static void main(String[] args) {
ListNode node1 = new ListNode("A");
ListNode node2 = new ListNode("B");
ListNode node3 = new ListNode("C");
ListNode node4 = new ListNode("D");
ListNode node5 = node2;//这里是有无环，不是有无重复的结点数值！！！！
ListNode node6 = node3;
node1.next = node2;
node2.next = node3;
node3.next = node4;
node4.next = node5;
node5.next = node6;
System.out.println(new Main().hasCycle(node1));//true

//        ListNode node1 = new ListNode("A");
//        ListNode node2 = new ListNode("B");
//        node1.next = node2;
//        System.out.println(new Main().hasCycle(node1));//false

//        ListNode node1 = new ListNode("A");
//        System.out.println(new Main().hasCycle(node1));//false

}
}