// Textbook fragment 07.31
/** Compute the value of an arithmetic expression tree. */
public class EvaluateExpressionTour extends EulerTour<ExpressionTerm, Integer> {
  public Integer execute(BinaryTree<ExpressionTerm> T) {
    init(T); // calls method of superclass
    return eulerTour(tree.root());  // returns the value of the expression
  }
  protected void visitRight(Position<ExpressionTerm> v, TourResult<Integer> r) {
    ExpressionTerm term = v.element();
    if (tree.isInternal(v)) {
      ExpressionOperator op = (ExpressionOperator) term;
      op.setOperands(r.left, r.right);
    }
    r.out = term.getValue();
  }
}