有关于链表实现搜索数据的有关问题

public class BST<Key extends Comparable<Key>, Value> {
    private Node root;             // root of BST

    
    private class Node {
        private Key key;           // sorted by key
        private Value val,val2,val3;        // associated data
        private Node left, right;  // left and right subtrees
        private Node mid;
        private int N;             // number of nodes in subtree

        
        SD sd=new SD();
        Queue<Value> ele = new Queue<Value>();
        public Node(Key key, Value val,Value val2,Value val3, int N) {
            this.key = key;
            this.sd.one = val;
            this.sd.two=val2;
            this.sd.three=val3;
            this.N = N;
        }
    }
  private SD get(Node x, Key key) {
        if (x == null) return null;
        int cmp = key.compareTo(x.key);
        if      (cmp < 0) return get(x.left, key);
        else if (cmp > 0) return get(x.right, key);
        else    {
                if(notmid(x.mid))
                    return x.sd;   
                else
                    return getmid(x.mid,key);
        }     
    }
    private SD getmid(Node x, Key key){ 
            x.sd.one=x.ele.dequeue();
            x.sd.two=x.ele.dequeue();
            x.sd.three=x.ele.dequeue();
            
            return get(x, key);
        
    }
    public void put(Key key, Value val,Value val2, Value val3) {
        if (val == null) { delete(key); return; }
        root = put(root, key, val,val2,val3);
        assert check();
    }

    private Node put(Node x, Key key, Value val,Value val2,Value val3) {
        if (x == null) 
        {   
            x.sd.one=val;
            x.sd.two=val2;
            x.sd.three=val3;
            
            return new Node(key, val,val2,val3, 1);
        }
        int cmp = key.compareTo(x.key);
        if      (cmp < 0) x.left  = put(x.left,  key, val,val2,val3);
        else if (cmp > 0) x.right = put(x.right, key, val,val2,val3);
        else    {
           
            x.mid=putsame(x.mid,key,val,val2,val3);
        }
        x.N = 1 + size(x.left) + size(x.right);
        return x;
    }
    private Node putsame(Node x,Key key,Value val,Value val2,Value val3){

           x.ele.enqueue(val);
           x.ele.enqueue(val2);
           x.ele.enqueue(val3);
        x.key=key;
        return x;
    }
    
    private boolean notmid(Node x){
        return x.mid.ele.isEmpty();
    }


      public void SearchData(int index,ArrayList<Student> x,String input){
             int size=x.size(); 
                    String[]    studentid=new String[size],
                                studentname=new String[size],
                                coursecode=new String[size],  
                                grade=new String[size];
                    Integer[]   code=new Integer[size];
                    
                    
                    String[] tem=new String[3];                
                              SD wer=new SD();
        BST<String, String> st = new BST<String,String>();           
        //size记录数组的大小    
        
                      String v1,v2,v3;
                                    

        //将Student类中的数据导出
        for(int i=0;i<size;i++){          
            studentid[i]=x.get(i).StudentID;
            studentname[i]=x.get(i).StudentName;
            coursecode[i]=x.get(i).ScoreCode;
            code[i]=x.get(i).Code;
        }
        //获取按钮中index，排序选中的元素，并将其他元素也进行排序
        //Quik.sort中需要排序的为第一个元素，第二第三第四为其他附属元素，跟着第一个元素位置的改动而改动
        switch(index){
                case 1:
                    //选中StudnetID进行排序,
                    Quick.sort(studentid,studentname,coursecode,code);
                    for(int i=0;i<x.size();i++)         grade[i]=code[i].toString();
                    for (int i = 0; i<x.size(); i++) {
                        String key = studentid[i];
                   
                            v1=studentname[i];
                            v2=coursecode[i];
                            v3=grade[i];
                       
                        st.put(key,v1,v2,v3);
                    }
                wer=st.get(input);
 
                System.out.println("StudentID is :"+input+"  StudentName is:"+wer.one+" soursecode is :"+wer.two+" score is :"+wer.three);       
                break;}
}