Case-specific Sorting of Strings

class Solution
{
    public:
    //Function to perform case-specific sorting of strings.
    string caseSort(string str, int n)
    {
        // your code here
        string allLower = "";
        string allUpper ="";

        for(int i = 0; i<n; i++){

            if(str[i] >='a' && str[i]<='z'){
                allLower = allLower + str[i];
            }
            else{
                allUpper = allUpper + str[i];
            }


        }

        sort(allLower.begin(), allLower.end());
        sort(allUpper.begin(), allUpper.end());

        int a = 0, b = 0;

        for(int i = 0; i<n; i++){

            if(str[i]>='a' && str[i]<='z'){
                str[i] = allLower[a];
                a++;
            }
            else{
                str[i] = allUpper[b];
                b++;
            }

        }

        return str;

    }
};