public int countVarOps(){
//pre:  this is a non-empty Arithmetic Expression Tree
//post: returns the number of non-numeric operands in the expression tree.
   // if the node is an empty node return 0 (no value or children)
   if (this.isEmpty()) {
      return 0;
   } else {
      Character root = (Character) this.getRoot();
      // check if this node is a letter. 
      // if so the total letters is 1 + total of children
      if (Character.isLetter(root.charValue()))
         return 1 + this.leftSubtree().countVarOps() + this.rightSubtree.countVarOps();
      else // else total = 0 + total of children
         return this.leftSubtree().countVarOps() + this.rightSubtree.countVarOps();
   }
}