Evaluate Reverse Polish Notation

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

Solution:

public class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<>();
        for(int i=0; i<tokens.length; i++){
            if(tokens[i].equals("+") || tokens[i].equals("-") || tokens[i].equals("*") || tokens[i].equals("/")){
                stack.push(calculate(stack.pop(), stack.pop(), tokens[i]));
            } else {
                stack.push(Integer.valueOf(tokens[i]));
            }
        }
        return stack.pop();
    }
    public int calculate(int b, int a, String op){
        int res = 0;
        switch(op){
            case "+" : res = a + b; break;
            case "-" : res = a - b; break;
            case "*" : res = a * b; break;
            case "/" : res = a / b; break;
        }
        return res;
    }
}