class Solution {

public:

    string decodeString(string s) {

        stack<string> chars;

        stack<int> nums;

        string res;

        int num = ;

        for (char c: s) {

            if (isdigit(c))

                num = num *  + (c - '');

            else if (isalpha(c)) {

                res += c;

            } else if (c == '[') {

                chars.push(res);

                nums.push(num);

                res = "";

                num = ;

            } else {

                string tmp = res;

                for (int i = ; i < nums.top() - ; i++)

                    res += tmp;

                res = chars.top() + res;

                chars.pop();

                nums.pop();

            }

        }

        return res;

    }

};