title: String toString method in BST (binary search tree)
body: <p>Why the next method does not print nothing but when i change the String s to array  , it works Properly? </p>
  
  <p>not working:</p>
  
  <pre><code>public String toString(){
      //In-Order - left,root,right.
      String s ="";
      toString(root,s);
      return s;
  }
  public void toString(BSTNode root,String s){
      if (root!=null){
          toString(root.left,s);
           s=s+","+ root.data;
           toString(root.right,s);
      }
  }
  </code></pre>
  
  <p>working:</p>
  
  <pre><code>public String toString(){
      //In-Order - left,root,right.
      String[] s =new String[1];
      s[0]="";
      toString(root,s);
      return s[0];
  }
  public void toString(BSTNode root,String[] s){
      if (root!=null){
          toString(root.left,s);
           s[0]=s[0]+","+ root.data;
           toString(root.right,s);
      }
  </code></pre>