BasicCalculatorIIClean.java

// Another thought is having 2 pass, first pass */, second pass +-
    // "1 + 2 * 3 / 2" = 4, pretty clean
    // "1 - 2 * 3 / 2" = -2
    public int calculateTwoPass(String s) {
        int len;
        if (s == null || (len = s.length()) == 0) {
            return 0;
        }
        Stack<Integer> stack = new Stack<Integer>();
        int num = 0;
        // This is more like the previous sign
        char sign = '+';
        for (int i = 0; i < len; i++) {
            if (Character.isDigit(s.charAt(i))) {
                num = num * 10 + s.charAt(i) - '0';
            }

            if ((!Character.isDigit(s.charAt(i)) && ' ' != s.charAt(i)) || i == len - 1) {
                if (sign == '-') {
                    stack.push(-num);
                }
                if (sign == '+') {
                    stack.push(num);
                }
                if (sign == '*') {
                    stack.push(stack.pop()*num);
                }
                if (sign == '/') {
                    stack.push(stack.pop()/num);
                }
                // reassign the current sign
                sign = s.charAt(i);
                num = 0;
            }
        }

        int re = 0;
        for (int i : stack){
            re += i;
        }
        return re;
    }