224. Basic Calculator

    /*
     * 224. Basic Calculator
     * 12.10 by Mingyang
     * 先把所有的该计算的都计算了，遇到符号就sign赋值，遇到数字就直接取结果
     * 最关键的是遇到括号以后,左边括号就把result和sign分别存stack里面，然后继续归零
     * 遇到右括号就把result和stack分别计算，最后return result
     * 注意：所有括号前面都有一个符号，所以都要重新赋值
     */
    public static int calculate(String s) {
        int len = s.length(), sign = 1, result = 0;
        Stack<Integer> stack = new Stack<Integer>();
        for (int i = 0; i < len; i++) {
            if (Character.isDigit(s.charAt(i))) {
                int sum = s.charAt(i) - '0';
                while (i + 1 < len && Character.isDigit(s.charAt(i + 1))) {
                    sum = sum * 10 + s.charAt(i + 1) - '0';
                    i++;
                }
                result += sum * sign;
            } else if (s.charAt(i) == '+')
                sign = 1;
            else if (s.charAt(i) == '-')
                sign = -1;
            else if (s.charAt(i) == '(') {
                stack.push(result);
                stack.push(sign);
                result = 0;
                sign = 1;
            } else if (s.charAt(i) == ')') {
                result = result * stack.pop() + stack.pop();
            }

        }
        return result;
    }