[LeetCode 150] Evaluate Reverse Polish Notation (Medium)

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

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

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

**Input:** ["2", "1", "+", "3", "*"]
**Output:** 9
**Explanation:** ((2 + 1) * 3) = 9

Example 2:

**Input:** ["4", "13", "5", "/", "+"]
**Output:** 6
**Explanation:** (4 + (13 / 5)) = 6

Example 3:

**Input:** ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
**Output:** 22
**Explanation:** 
  ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

class Solution {
    public int evalRPN(String[] tokens) {
        if (tokens == null || tokens.length == 0) {
            return 0;
        }
        
        Stack tracker = new Stack<> ();
        int result = 0;
        
        for (String token : tokens) {
            if (tracker.size () >= 2 && isOperators (token)) {
                int secondNumber = tracker.pop ();
                int firstNumber = tracker.pop ();
                
                if (token.equals ("+")) {
                    tracker.push (firstNumber + secondNumber);
                } else if (token.equals ("-")) {
                    tracker.push (firstNumber - secondNumber);
                } else if (token.equals ("*")) {
                    tracker.push (firstNumber * secondNumber);
                } else if (token.equals ("/")) {
                    tracker.push (firstNumber / secondNumber);
                }
            } else {
                tracker.push (Integer.valueOf (token));
            }
        }
        
        return (int) (tracker.pop ());
    }
    
    public boolean isOperators (String operator) {
        return operator.equals ("+") || operator.equals ("-") || operator.equals ("*") || operator.equals ("/");
    }
}