class Solution {

    static List<String> OPERATORS = Arrays.asList("+", "-", "*", "/");

    public int evalRPN(String[] tokens) {
        Stack<Long> stack = new Stack<Long>();
        for (String token : tokens) {
            if (isOperator(token)) {
                Long operandRight = stack.pop();
                Long operandLeft = stack.pop();
                stack.push(eval(operandLeft, operandRight, token.charAt(0)));
            } else {
                stack.push(Long.parseLong(token));
            }
        }
        return stack.pop().intValue();
    }

    boolean isOperator(String token) {
        return OPERATORS.contains(token);
    }

    Long eval(Long operandLeft, Long operandRight, char operator) {
        switch (operator) {
            case '+':
                return operandLeft + operandRight;
            case '-':
                return operandLeft - operandRight;
            case '*':
                return operandLeft * operandRight;
            case '/':
                return operandLeft / operandRight;
            default:
                throw new IllegalStateException();
        }
    }

}