class Solution {
    public String decodeString(String s) {

        Stack<Integer> countStack = new Stack<>();
        Stack<String> valueStack = new Stack<>();
        StringBuilder sb = new StringBuilder();

        int idx = 0;
        String res = "";
        while(idx<s.length()){
            if(Character.isDigit(s.charAt(idx))){
                int cnt = 0;
                while(idx<s.length() && Character.isDigit(s.charAt(idx))){
                    cnt = cnt*10+(s.charAt(idx)-'0');
                    idx++;
                }
                countStack.add(cnt);
            }else if(s.charAt(idx)=='['){
                valueStack.add(res);
                res="";
                idx++;
            }else if(s.charAt(idx)==']'){
                StringBuilder tmp = new StringBuilder(valueStack.pop());
                int repeat = countStack.pop();
                for(int i=0;i<repeat;++i)
                    tmp.append(res);
                res = tmp.toString();
                idx++;
            }else
                res+=s.charAt(idx++);
        }
        return res;
    }
}