#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <functional>

std::string decodeString(std::string s);
std::string createStringForStack(const std::string &s, const size_t sSize, int leftIdx, std::function<bool(char)> predicate);
bool is_number(const std::string &s);

int main()
{
   std::string s = "3[a2[c]]";
   std::string decodedString = decodeString(s);
   std::cout << decodedString << std::endl;
}

std::string decodeString(std::string s)
{
   std::stack<std::string> st;
   std::stack<std::string> stKeepClosedP;
   std::string result = "";
   std::string currentString = "";

   const size_t sSize = s.size();

   auto predicateStringBuilder = [](char c)
   { return ('a' <= c && 'z' >= c); };

   auto predicateNumberBuilder = [](char c)
   { return ('0' <= c && '9' >= c); };

   for (int i = 0; i < sSize; ++i)
   {
      if (s[i] == ']' || s[i] == '[')
         st.push(std::string{s[i]});
      else if (s[i] >= 'a' && s[i] <= 'z')
         st.push(createStringForStack(s, sSize, i, predicateStringBuilder));
      else
         st.push(createStringForStack(s, sSize, i, predicateNumberBuilder));
   }

   std::string buffer = ""; // change it to ostringstream
   // std::ostringstream oss;
   while (!st.empty())
   {
      currentString = st.top();
      st.pop();

      while (!st.empty() && st.top() == "]")
      {
         st.pop();
         stKeepClosedP.push("]");
      }

      if (currentString[0] == ']')
      {
         buffer = st.top();
         // oss << st.top();

         st.pop();
         st.pop();                         // popping the "["
         int bufferTimes = stoi(st.top()); // getting the number into 'stoi' string to integer function
         st.pop();                         // remove the number we don't need it anymore.
         st.push(buffer);                  // just to save new variable
         // st.push(oss.str()); // just to save new variable
         // oss.clear();
         buffer = "";
         for (int i = 0; i < bufferTimes; ++i)
         {
            buffer += st.top();
            // oss << st.top();
         }
         st.pop(); // popping the original string it was

         // std::ostringstream keep;
         while (!st.empty() && st.top() != "[" && st.top() != "]" && !is_number(st.top()))
         {
            // keep << oss.str();
            // oss.clear();
            buffer = st.top() + buffer;

            // oss << st.top();
            // oss << keep.str();
            // keep.clear();
            st.pop();
         }
         // st.push(oss.str());
         // oss.clear();
         st.push(buffer);

         while (!stKeepClosedP.empty())
         {
            stKeepClosedP.pop();
            st.push("]");
         }
      }
      else
      {
         result = buffer + result;
         // result = oss.str() + result;
      }
   }

   return result;
}

std::string createStringForStack(const std::string &s, const size_t sSize, int leftIdx, std::function<bool(char)> predicate)
{
   int rightIdx = leftIdx;
   for (; rightIdx < sSize && predicate(s[rightIdx]); ++rightIdx)
   {
   }
   return s.substr(leftIdx, rightIdx - leftIdx);
}

bool is_number(const std::string &s)
{
   return !s.empty() && std::all_of(s.begin(), s.end(), ::isdigit);
}