Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> s = new Stack<Integer>();
        String operators = "+-*/";
        for(String token : tokens){
            if(!operators.contains(token)){
                s.push(Integer.valueOf(token));
                continue;
            }

            int a = s.pop();
            int b = s.pop();
            if(token.equals("+")) {
                s.push(b + a);
            } else if(token.equals("-")) {
                s.push(b - a);
            } else if(token.equals("*")) {
                s.push(b * a);
            } else {
                s.push(b / a);
            }
        }
        return s.pop();
    }
}

class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<>();

        for (int i = 0; i < tokens.length; i++) {

            String str = tokens[i];

            if (str.equals("+") || str.equals("*") || str.equals("/") || str.equals("-"))             {
                int a2 = stack.pop();
                int a1 = stack.pop();
                if (str.equals("+"))
                    stack.push(a1 + a2);
                else if (str.equals("-"))
                    stack.push(a1 - a2);
                else if (str.equals("*"))
                    stack.push(a1 * a2);
                else if (str.equals("/"))
                    stack.push(a1 / a2);
            } else stack.push(Integer.parseInt(str));

        }

        return stack.pop();
    }
}

class Solution {
    public int evalRPN(String[] tokens) {
        Deque<Integer> intermediateResults = new LinkedList<>();
        for(String s : tokens){
            if("+-*/".contains(s)){
                int y = intermediateResults.removeLast();
                int x = intermediateResults.removeLast();
                switch(s){
                    case "+":
                        intermediateResults.add(x + y); break;  
                    case "-":
                        intermediateResults.add(x - y); break;
                    case "*":
                        intermediateResults.add(x * y); break;  
                    case "/":
                        intermediateResults.add(x / y); break;    
                }
            } else {
                intermediateResults.add(Integer.parseInt(s));
            }
        }
        return intermediateResults.poll();
    }
}