Main.java

rpn

package org.zakky.rpn;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Stack;

public class Main {

    public static void main(String[] args) throws Exception {
        final BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        String line = reader.readLine();
        final Stack<Long> s = new Stack<Long>();
        final String[] inputs = line.split(" +");
        for (String input : inputs) {
            if (Character.isDigit(input.charAt(0))) {
                s.push(Long.valueOf(input));
            } else if (input.charAt(0) == '+') {
                s.push(s.pop() + s.pop());
            } else if (input.charAt(0) == '-') {
                long v2 = s.pop();
                long v1 = s.pop();
                s.push(v1 - v2);
            } else if (input.charAt(0) == '*') {
                s.push(s.pop() * s.pop());
            } else if (input.charAt(0) == '/') {
                long v2 = s.pop();
                long v1 = s.pop();
                s.push(v1 / v2);
            }
        }
        System.out.println((s.size() == 1) ? ("The answer is: " + s.pop())
                : ("invalid RPN expression: " + line));
    }
}