棒！添加注解版！

   public static String parse(String str){
        StringBuilder sb = new StringBuilder();
        LinkedList<Integer> len = new LinkedList<>();
        String temp = ""; // 记录括号内的字符串

        // 针对字符串，处理顺序依次为：去括号，统计出现次数，拼接结果
        for(int i=0;i<str.length();i++){
            char c = str.charAt(i);
            if(c == '('){
                len.add(sb.length());
                continue;
            }
            if(c == ')'){
                // 移除上一个括号开始时的下标
                int newLeftIndex = len.remove(len.size()-1);
                temp = sb.substring(newLeftIndex);
                i++;
            }
            // 处理数字
            int start = i;
            while (i<str.length() && str.charAt(i)>='0' && str.charAt(i)<='9'){
                i++;
            }
            // 获取字符后面的数字
            if(i-start>=1){
                int count = Integer.valueOf(str.substring(start,i));
                for(int k=0;k<count-1;k++){
                    if (temp==""){
                        sb.append(str.charAt(start-1));
                    }else {
                        sb.append(temp);
                    }
                }
                temp = "";
                i--;//若存在数字，此时i指向的字符并没有经过上述判断，所以需要减去1.
            }else {
                sb.append(str.charAt(i));
            }
        }
        return sb.toString();
    }