class Solution {
public:
    string decodeString(string s) {
        int len = s.size();
        stack<string> st;
        stack<int> nt;
        for (int i = 0; i < len; i++) {
            if (s[i] >= '0' && s[i] <= '9') {
                string ss = "";
                int j = i;
                for ( ; s[j] >= '0' && s[j] <= '9'; j++)
                    ss += s[j];
                nt.push(stoi(ss));
                i = j - 1;
            } else if (s[i] == ']') {
                string ss = "";
                while (st.top() != "[") {
                    ss = st.top() + ss;
                    st.pop();
                }
                st.pop();
                ss.reserve();
                string t = "";
                int num = nt.top();
                nt.pop();
                for (int i = 0; i < num; i++)
                    t += ss;
                st.push(t);
            } else {
                string ss = "";
                ss += s[i];
                st.push(ss);
            }
        }
        string ss = "";
        while (!st.empty()) {
            ss = st.top() + ss;
            st.pop();
        }
        return ss;
    }
};