decodestring.cpp

#include <bits/stdc++.h>
using namespace std;

string decode(string str)
{
	stack<int> numstack;
	stack<char> charstack;
	string nstr;
	for (int i = 0; str[i] != '\0'; i++)
	{
		if (isdigit(str[i]))
		{
			nstr += str[i];
		}
		else if (str[i] == '[')
		{
			charstack.push(str[i]);
			int no = stoi(nstr);
			numstack.push(no);
			nstr = "";
		}
		else if (str[i] == ']')
		{
			string poppedstr;
			char c = charstack.top();
			charstack.pop();
			while (c != '[')
			{
				poppedstr += c;
				c = charstack.top();
				charstack.pop();
			}
			reverse(poppedstr.begin(), poppedstr.end());
			int no = numstack.top();
			numstack.pop();
			for (int x = 0; x < no; x++)
				for (int j = 0; j < poppedstr.length(); j++)
					charstack.push(poppedstr[j]);
		}
		else
		{
			charstack.push(str[i]);
		}
	}
	string ans;
	while (!charstack.empty())
	{

		ans += charstack.top();
		charstack.pop();
	}
	reverse(ans.begin(), ans.end());
	return ans;
}

int main()
{
	string str;
	cout << "Enter the string to be decoded : ";
	cin >> str;
	string ans = decode(str);
	cout << ans;
}