ReversePolishNotation.java

import java.util.Stack;

/**
 * @author humayun
 *
 * https://leetcode.com/problems/evaluate-reverse-polish-notation/
 *
 */
public class ReversePolishNotation {

    public static void main(String[] args) {
        String [] tokens = new String[] {"4", "13", "5", "/", "+"}; // 4 + 13 / 5
        System.out.println(new ReversePolishNotation().evalRPN(tokens));
    }

    public int evalRPN(String[] tokens) {
        String operators = "+-*/";

        Stack<String> stack = new Stack<String>();
        for (String t : tokens) {
            if (!operators.contains(t)) {
                stack.push(t);
            } else {
                int a = Integer.valueOf(stack.pop());
                int b = Integer.valueOf(stack.pop());
                switch (t) {
                    case "+":
                        stack.push(String.valueOf(a + b));
                        break;
                    case "-":
                        stack.push(String.valueOf(b - a));
                        break;
                    case "*":
                        stack.push(String.valueOf(a * b));
                        break;
                    case "/":
                        stack.push(String.valueOf(b / a));
                        break;
                }
            }
        }

        return Integer.valueOf(stack.pop());
    }

}