using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        //input: "3[a2[c]]"

        string inputString= "3[a]2[bc]";

        //mm(inputString);

        List<int> openIndexes=new List<int>();

        Console.WriteLine("inputString "+inputString);

        while(inputString.Contains('['))

        {

            inputString = formatString(inputString,openIndexes);

        }

        //inputString = formatString(inputString,openIndexes);

        Console.WriteLine("outString "+inputString);

        //Console.WriteLine(" -------------------- ");

        //inputString = formatString(inputString,openIndexes);

    }

​

    public static string formatString(string inputString,List<int> openIndexes)

    {

        //Console.WriteLine("openIndexes Count "+ openIndexes.Count());

        int openIndex,closeIndex;

        for(int i=0;i<inputString.Length;i++)

        {           

            if(inputString[i]=='[')

            {

                openIndexes.Add(i);

            }

            else if(inputString[i]==']')

            {

                if(openIndexes.Count==0)

                {

                    Console.WriteLine("Wrong Format");

                }

                else

                {

                    closeIndex=i;

                    openIndex=openIndexes.Last();

                    //Console.WriteLine("closeIndex " + closeIndex+"--- openIndex "+openIndex);

                    string repeatString = inputString.Substring(openIndex+1,closeIndex-(openIndex+1));

                    //Console.WriteLine("repeatString " + repeatString);

                    int repeatNumber=Convert.ToInt32( inputString.Substring(openIndex-1,1));

                    //Console.WriteLine("repeatNumber " + repeatNumber);

                    string newReplaceString="";

                    string replaceString=inputString.Substring(openIndex-1,closeIndex-(openIndex-2));

                    //Console.WriteLine("replaceString " + replaceString);

                    for(int j=0;j<repeatNumber;j++)

                    {

                        newReplaceString=newReplaceString+repeatString;

                    }

                    //Console.WriteLine("newReplaceString " + newReplaceString);

                    inputString=inputString.Replace(replaceString,newReplaceString);

                    openIndexes.Remove(openIndex);

                    break;

                }

            }

        }

        return inputString;

    }

}