20220608 other: evaluation of inverse Polish expression

        Title Description ： according to   Reverse Polish notation , Find the value of the expression . Valid operators include  +、-、*、/ . Each operand can be an integer , It can also be another inverse Polish expression . Be careful   The division between two integers retains only the integer part .

        coded ：

    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<>();
        for (String token : tokens) {
            if (noDigit(token)) {
                Integer b = stack.pop();
                Integer a = stack.pop();
                stack.push(calc(a, b, token));
                continue;
            }
            stack.push(Integer.parseInt(token));
        }
        return stack.peek();
    }

    private boolean noDigit(String s) {
        return "+".equals(s) || "-".equals(s) || "*".equals(s) || "/".equals(s);
    }

    private int calc(int a, int b, String c) {
        switch (c) {
            case "+":
                return a + b;
            case "-":
                return a - b;
            case "*":
                return a * b;
            case "/":
                return a / b;
            default:
                return 0;
        }
    }