/**
 * A demo of evaluating an expression stored as an expression tree.
 */
public class ExpressionTreeDemo {

    /**
     * Build and evaluate a binary tree for (((4+3)*(10-5))/2).
     */
    public static void main(String[] args) {

        BinaryTree<String> four = new BinaryTree<String>("4");
        BinaryTree<String> three = new BinaryTree<String>("3");
        BinaryTree<String> ten = new BinaryTree<String>("10");
        BinaryTree<String> five = new BinaryTree<String>("5");
        BinaryTree<String> two = new BinaryTree<String>("2");

        BinaryTree<String> plus = new BinaryTree<String>("+", four, three);
        BinaryTree<String> minus = new BinaryTree<String>("-", ten, five);
        BinaryTree<String> times = new BinaryTree<String>("*", plus, minus);
        BinaryTree<String> divide = new BinaryTree<String>("/", times, two);

        System.out.println(evaluate(divide));
    }

    /**
     *  Evaluate an expression stored in a binary expression tree
     */
    public static double evaluate(BinaryTree<String> tree) {
        if (tree.isLeaf()) {
            return Double.parseDouble(tree.value());
        } else {
            double leftVal = evaluate(tree.left());
            double rightVal = evaluate(tree.right());
            String op = tree.value();
            if (op.equals("+")) {
                return leftVal + rightVal;
            } else if (op.equals("-")) {
                return leftVal - rightVal;
            } else if (op.equals("*")) {
                return leftVal * rightVal;
            } else if (op.equals("/")) {
                return leftVal / rightVal;
            } else {
                throw new IllegalArgumentException("unrecognized operator " + op);
            }
        }
    }

}