Why am i getting Crash with this solution in Java? Thank you.

public static Vector<Integer> res=new Vector<Integer>();
    public static class Tree{
        public Tree l;
        public Tree r;
        int val;
        Tree(int v){
            this.val=v;
            this.l=null;
            this.r=null;
        }
        public static void add(Tree t,int v){
            if(v<t.val){
                if(t.l==null){
                    t.l=new Tree(v);
                }
                else{
                    add(t.l,v);
                }
            }
            else if(v>t.val){
                if(t.r==null){
                    t.r=new Tree(v);
                }
                else{
                    add(t.r,v);
                }
            }
        }
        public static void rightSearch(Tree t){
            if(t.r!=null)
                rightSearch(t.r);
            if(t.l!=null)
                rightSearch(t.l);
            res.add(t.val);
        }
    }
    public static void main(String[] args) throws IOException {
        BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
        PrintWriter out=new PrintWriter(System.out);
        int n=Integer.parseInt(in.readLine());
        String[] input=in.readLine().split(" ");
        Tree t=new Tree(Integer.parseInt(input[n-1]));
        for(int i=n-2; i>=0; i--){
            Tree.add(t,Integer.parseInt(input[i]));
        }
        Tree.rightSearch(t);
        for(int i=0; i<res.size(); i++){
            out.print(res.get(i)+" ");
        }
        out.close();
    }