decode_the_string.cpp

#include<bits/stdc++.h> 
using namespace std; 
string decode(string str) 
{ 
    stack<int> integerstack; 
    stack<char> stringstack; 
    string temp = "", result = ""; 
    for (int i = 0; i < str.length(); i++) 
    { 
        int count = 0; 
        if (str[i] >= '0' && str[i] <='9') 
        { 
            while (str[i] >= '0' && str[i] <= '9') 
            { 
                count = count * 10 + str[i] - '0'; 
                i++; 
            } 
  
            i--; 
            integerstack.push(count); 
        } 
        else if (str[i] == ']') 
        { 
            temp = ""; 
            count = 0; 
  
            if (! integerstack.empty()) 
            { 
                count = integerstack.top(); 
                integerstack.pop(); 
            } 
  
            while (! stringstack.empty() && stringstack.top()!='[' ) 
            { 
                temp = stringstack.top() + temp; 
                stringstack.pop(); 
            } 
  
            if (! stringstack.empty() && stringstack.top() == '[') 
                stringstack.pop(); 
            for (int j = 0; j < count; j++) 
                result = result + temp; 
            for (int j = 0; j < result.length(); j++) 
                stringstack.push(result[j]); 
  
            result = ""; 
        } 
        else if (str[i] == '[') 
        { 
            if (str[i-1] >= '0' && str[i-1] <= '9') 
                stringstack.push(str[i]); 
  
            else
            { 
                stringstack.push(str[i]); 
                integerstack.push(1); 
            } 
        } 
  
        else
            stringstack.push(str[i]); 
    }
    while (! stringstack.empty()) 
    { 
        result = stringstack.top() + result; 
        stringstack.pop(); 
    } 
  
    return result; 
} 
int main() 
{ 
	int t;
	cin>>t;
	while(t--){
	string str;
    cin>>str;
    cout << decode(str) << endl; 
	}
    return 0; 
}