Basic calculator II for leetcode topic analysis

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7

" 3/2 " = 1

" 3+5 / 2 " = 5

Note: Do not use the eval built-in library function.

Go through it twice , The first time I traverse , The multiplication and division symbols are used to calculate ; Second traversal , Calculating addition and subtraction symbols .

    public int calculate(String s) {
        if (s == null || s.length() == 0) {
            return 0;
        }
        Stack<Integer> stack = new Stack<Integer>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (Character.isDigit(c)) {
                int val = c - '0';
                //  Take out the number 
                while (i + 1 < s.length() && Character.isDigit(s.charAt(i + 1))) {
                    val = val * 10 + s.charAt(++i) - '0';
                }
                //  At the top of the stack  * /  Operator , Calculation 
                if (!stack.isEmpty()
                        && (stack.peek() == 2 || stack.peek() == 3)) {
                    int sign = stack.pop();
                    int op = stack.pop();
                    int rt = 0;
                    if (sign == 2) {
                        rt = op * val;
                    } else {
                        rt = op / val;
                    }
                    stack.push(rt);
                } else {
                    stack.push(val);
                }
            } else if (c == ' ') {
                continue;
            } else {
                switch (c) {
                case '+':
                    stack.push(0);
                    break;
                case '-':
                    stack.push(1);
                    break;
                case '*':
                    stack.push(2);
                    break;
                case '/':
                    stack.push(3);
                    break;
                }
            }
        }
        if (stack.isEmpty()) {
            return 0;
        }
        //  Because it needs to be calculated from left to right , So we need to reverse
        Collections.reverse(stack);
        //  Calculation +-
        int rt = stack.pop();
        while (!stack.isEmpty()) {
            int sign = stack.pop();
            int op = stack.pop();
            if (sign == 0) {
                rt += op;
            } else {
                rt -= op;
            }
        }
        return rt;
    }