public class Node{
public int val;
public Node next;
}
public boolean isHasLoop(Node head){
if(head == null) return false;
Node slow = head.next;
if(slow == null) return false;
Node fast = slow.next;
while(fast != null && slow != null){
if(fast == slow) return true;
slow = slow.next;
fast = fast.next;
//快指针走两步时,要注意避免空指针异常
if(fast != null) fast = fast.next;
}
return false;
}