public ListNode FindKthToTail(ListNode head,int k) {
        ListNode cur = head;
        ListNode sign = head;
        int count = 0;
        while(cur != null){
            count++;
            cur = cur.next;
        }
        int n = count - k;
        if(head == null || n < 0){
            return null;
        }
        for(int i =0; i<n; i++){
            sign = sign.next;
        }
        return sign;
    }
}