title: Method isEmpty for binary tree issue
body: <p>I am trying to write myself the isEmpty method for a binary tree but I am having a problem. So this is the method that I am using.</p>
  
  <pre><code>public boolean isEmpty(){
  if(root == null) return true;
  else return false;
  }
  </code></pre>
  
  <p>When I add only one element, and then remove this element, and call isEmpty, I do not get true, but false. </p>
  
  <p>Is there somethng wrong with my implementation?</p>
  
  <hr>
  
  <p>So this is the remove method:</p>
  
  <pre><code>  /**
      * Internal method to remove from a subtree.
      * @param x the item to remove.
      * @param t the node that roots the tree.
      * @return the new root.
      * @throws ItemNotFoundException if x is not found.
      */
      protected BinaryNode&lt;AnyType&gt; remove( AnyType x, BinaryNode&lt;AnyType&gt; t )
      {
      if( t == null )
      throw new ItemNotFoundException( x.toString( ) );
      if( x.compareTo( t.element ) &lt; 0 )
      t.left = remove( x, t.left );
      else if( x.compareTo( t.element ) &gt; 0 )
      t.right = remove( x, t.right );
      else if( t.left != null &amp;&amp; t.right != null ) // Two children
      {
      t.element = findMin( t.right ).element;
      t.right = removeMin( t.right );
      }
      else
      t = ( t.left != null ) ? t.left : t.right;
      return t;
      }
  </code></pre>
  
  <p>and this is the removeMin method that the remove method uses:</p>
  
  <pre><code>        /**
           * Internal method to remove minimum item from a subtree.
           * @param t the node that roots the tree.
           * @return the new root.
           * @throws ItemNotFoundException if t is empty.
           */
           protected BinaryNode&lt;AnyType&gt; removeMin( BinaryNode&lt;AnyType&gt; t )
           {
           if( t == null )
          throw new ItemNotFoundException( );
          else if( t.left != null )
          {
          t.left = removeMin( t.left );
          return t;
          }
          else
          return t.right;
          }
  </code></pre>