思路：采用快慢指针，若有环 快慢指针一定会在某处相等

public boolean checkCycle(ListNode head){

    if(head==null)
        return fasle;
    ListNode slow = head;
    ListNode fast = head;
    
    while(fast!=null&&fast.next!=null){
        slow = slow.next;
        fast = fast.next.next;
        if(slow==fast)
            return true;
    }
    return false;
}