双栈法

  public String decodeString(String s) {

        String res = "";
        Stack<Integer> countStack = new Stack<>();
        Stack<String> resStack = new Stack<>();
        int idx = 0;
        while (idx < s.length()){
            char cur = s.charAt(idx);
            //处理数字
            if(Character.isDigit(cur)){
                StringBuffer ret = new StringBuffer();
                while (Character.isDigit(s.charAt(idx))){
                    ret.append(s.charAt(idx++));
                }
                countStack.push(Integer.parseInt(ret.toString()));
            } else if (cur == '[') {
                //处理[
                resStack.push(res);
                res = "";
                idx++;
            }else if(cur == ']'){
                //处理']',处理相匹配的
                StringBuffer temp = new StringBuffer(resStack.pop());
                int repeatTimes = countStack.pop();
                for(int i=0;i<repeatTimes;i++){
                    temp.append(res);
                }
                res = temp.toString();
                idx++;
            }else{
                //处理普通字符
                res+=s.charAt(idx++);
            }

        }
        return res;

    }