判断链表是否有环

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;
}