394. Decode String

https://leetcode.com/problems/decode-string/description/

class Solution {
public:
    string decodeString(const string &s, int &idx)
    {
        string res;
        int num = 0;
        for (; idx < s.length(); idx++)
        {
            if (isdigit(s[idx]))
            {
                num = num * 10 + s[idx] - '0';
            }
            else if (s[idx] == '[')
            {
                idx++;
                string sub = decodeString(s, idx);
                for (int i = 0; i < num; i++)
                {
                    res += sub;
                }
                num = 0;
            }
            else if (s[idx] == ']')
            {
                return res;
            }
            else
            {
                res.push_back(s[idx]);
            }
        }
        return res;
    }
    string decodeString(string s) {
        int idx = 0;
        return decodeString(s, idx);
        /*
        stack<int> si;
        stack<string> ss;
        string cur_str="";
        int cur_num = 0;
        for (int idx = 0; idx < s.length(); idx++)
        {
            if (isdigit(s[idx]))
                cur_num = cur_num * 10 + s[idx] - '0';
            else if (s[idx] == '[')
            {
                si.push(cur_num);
                ss.push(cur_str);
                cur_str = "";
                cur_num = 0;
            }
            else if (s[idx] == ']')
            {
                int prev_num = si.top(); 
                string prev_str = ss.top();
                si.pop();
                ss.pop();
                
                while (prev_num-- > 0)
                    prev_str += cur_str;
                
                cur_str = prev_str;
                cur_num = 0;
            }
            else
                cur_str.push_back(s[idx]);
        }
        return cur_str;
        */
            
        /*
        stack<int> si;
        stack<string> st;
        int idx = 0;
        while (idx < s.length())
        {
            if (isdigit(s[idx]))
            {
                int num = 0;
                while (isdigit(s[idx]))
                {
                    num = num * 10 + s[idx] - '0';
                    idx++;
                }
                si.push(num);
            }
            else if (s[idx] == ']')
            {
                string cur;
                while (st.top() != "[")
                {
                    cur = st.top() + cur;
                    st.pop();
                }
                st.pop();   // pop [
                
                int num = si.top(); si.pop();
                string curRes;
                while (num-- > 0)
                    curRes += cur;
                
                st.push(curRes);
                
                idx++;
            }
            else if (s[idx] == '[')
            {
                st.push("[");
                idx++;
            }
            else
            {
                string cur;
                while (s[idx] != 0 && !isdigit(s[idx]) && s[idx] != '[' && s[idx] != ']')
                    cur.push_back(s[idx++]);
                st.push(cur);
            }
        }
        string res;
        while (!st.empty())
        {
            res = st.top() + res;
            st.pop();
        }
        return res;
        */
    }
};