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.

Example

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

Analysis:
Use a stack to save the numbers, once an operator is found, pop two numbers from stack, and push the result back to stack. Note: make sure the order of the operands.

public class Solution {
    public int evalRPN(String[] tokens) {
        if (tokens == null || tokens.length == 0) return 0;
        Stack<Integer> stack = new Stack<>();
        
        for (String token : tokens) {
            if (isOperator(token)) {
                if (stack.size() < 2) return 0; 
                int value1 = stack.pop();
                int value2 = stack.pop();
                if (token.equals("+")) {
                    stack.push(value1 + value2);
                } else if (token.equals("-")) {
                    stack.push(value2 - value1);
                } else if (token.equals("*")) {
                    stack.push(value1 * value2);
                } else {
                    if (value1 == 0) {
                        return Integer.MAX_VALUE;
                    }
                    stack.push(value2 / value1);
                }
            } else {
                stack.push(Integer.parseInt(token));
            }
        }
        return stack.pop();
    }
    
    private boolean isOperator(String token) {
        return token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/");
    }
}