/*
 *  Each node in the tree holds a String that is either a binary operator
 *  (restricted to "+", "-", "*", "/") or can be treated as an integer value.
 */
public class ExpressionTreeDemo {


    public static void main(String[] args) {

        /* build and evaluate the binary tree for ((4+3)*(10-5))/2 */
        /* build it from the bottom up */
        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");

        /* build the actual tree structure */
        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);

        /* what's the answer */
        System.out.println(evaluate(divide));
    }

    public static int evaluate(BinaryTree<String> tree) {
        if (tree.isLeaf()) {
           return Integer.parseInt(tree.value()); 
        } else {
            int left = evaluate(tree.left());
            int right = evaluate(tree.right());
            String op = tree.value();
            if (op.equals("+")) {
                return left + right;
            } else if (op.equals("-")) {
                return left - right;
            } else if (op.equals("*")) {
                return left * right;
            } else if (op.equals("/")) {
                return left / right;
            } else {
                throw new IllegalStateException("bad op: " + op);
            }
        }
    }
    
}