3. Evaluate Reverse Polish Notation

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return an integer that represents the value of the expression.

Note that:

The valid operators are '+', '-', '*', and '/'.
Each operand may be an integer or another expression.
The division between two integers always truncates toward zero.
There will not be any division by zero.
The input represents a valid arithmetic expression in a reverse polish notation.
The answer and all the intermediate calculations can be represented in a 32-bit 
integer.

Example 1:

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

class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<>();
 
        for(String token : tokens) {
            if(token.equals("+")) {
                stack.push(stack.pop() + stack.pop());
            } else if(token.equals("-")) {
                int a = stack.pop();
                int b = stack.pop();
                stack.push(b-a);
            } else if(token.equals("*")) {
                stack.push(stack.pop() * stack.pop());
            } else if(token.equals("/")) {
                int a = stack.pop();
                int b = stack.pop();
                stack.push(b/a);
            } else {
                stack.push(Integer.parseInt(token));
            }
        }
 
        return stack.peek();
    }
}