求倒数第K个节点

public ListNode FindKthToTail(ListNode head,int k) {
         /*倒数第k个节点，即正数第len-k+1个节点*/
         if(head==null) return null;
         ListNode cur=head;
         int len=0,count=0;
         while(cur!=null) {
             len++;
             cur=cur.next;
         }
         if(k>len) return null;

         cur=head;
         while(cur!=null) {
             count++;
             if(count==len-k+1) return cur;
             cur=cur.next;
         }
         return null;
        }