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 Example 2:

Input: tokens = [“4”,“13”,“5”,"/","+"] Output: 6 Explanation: (4 + (13 / 5)) = 6 Example 3:

Input: tokens = [“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

<解題>

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