Exercise1311_EvaluatePostfix.java

public class Exercise1311_EvaluatePostfix {
    public static void main(String[] args) {
        Stack<String> ops = new Stack<String>();
        Stack<Double> vals = new Stack<Double>();
        while (!StdIn.isEmpty())
        {
            String s = StdIn.readString();
            if (s.equals("("));
            else if (s.equals("+"))
                ops.push(s);
            else if (s.equals("-"))
                ops.push(s);
            else if (s.equals("*"))
                ops.push(s);
            else if (s.equals("/"))
                ops.push(s);
            else if (s.equals("sqrt")) ops.push(s);
            else if (s.equals(")"))
            {
                String op = ops.pop();
                double v = vals.pop();
                if (op.equals("+"))
                    v = vals.pop() + v;
                else if (op.equals("-"))
                    v = vals.pop() - v;
                else if (op.equals("*"))
                    v = vals.pop() * v;
                else if (op.equals("/"))
                    v = vals.pop() / v;
                else if (op.equals("sqrt")) v = Math.sqrt(v);
                vals.push(v);
            }
            else vals.push(Double.parseDouble(s));
        }
        StdOut.println(vals.pop());
    }
}