Evaluar el ejemplo de código de notación polaca inversa

Ejemplo 1: evaluar la notación polaca inversa gfg

public class Test {
 
	public static void main(String[] args) throws IOException {
		String[] tokens = new String[] { "2", "1", "+", "3", "*" };
		System.out.println(evalRPN(tokens));
	}
 
	public static int evalRPN(String[] tokens) {
		int returnValue = 0;
		String operators = "+-*/";
 
		Stack<String> stack = new Stack<String>();
 
		for (String t : tokens) {
			if (!operators.contains(t)) { //push to stack if it is a number
				stack.push(t);
			} else {//pop numbers from stack if it is an operator
				int a = Integer.valueOf(stack.pop());
				int b = Integer.valueOf(stack.pop());
				switch (t) {
				case "+":
					stack.push(String.valueOf(a + b));
					break;
				case "-":
					stack.push(String.valueOf(b - a));
					break;
				case "*":
					stack.push(String.valueOf(a * b));
					break;
				case "https://foroayuda.es/":
					stack.push(String.valueOf(b / a));
					break;
				}
			}
		}
 
		returnValue = Integer.valueOf(stack.pop());
 
		return returnValue;
	}
}

Ejemplo 2: evaluar la notación polaca inversa gfg

["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "https://foroayuda.es/", "+"] -> (4 + (13 / 5)) -> 6

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