#include <bits/stdc++.h>
using namespace std;
int main()
{
    int t;
    cin>>t;
    while(t--)
    {
        int n;
        cin>>n;
        string s;
        cin>>s;
        string a= "";//taking an empty string to store the output
        int i= n-1;
        while(i>= 0)//traversing the string from the rear
        {
            if(s[i] != 'a' && s[i] != 'e')//if its not a vowel, concatinating the last three elements to the "a" string
            {
                a= a+s[i];
                a= a+s[i-1];
                a= a+s[i-2];
                a= a+'.';
                i= i-3;
            }
            else//else, concatinating the last two elements to the "a" string
            {
                a= a+s[i];
                a= a+s[i-1];
                a= a+'.';
                i= i-2;
            }
        }
        a.pop_back();//popping out the last "."
        reverse(a.begin(), a.end());//reversing the output since I traversed from the rear
        cout<<a<<endl;
    }
    return 0;
}

Hey CF!, I came up with a solution to the below mentioned question and my code is getting a tle. What could be the problem? click here to view the problem

I must also mention that the first and the second hidden test cases are running fine but the third one is where the problem is. The constraints for "n" is 2*10^5, but my code is fulfilling it.

please let me know what is wrong in this code, thanking you.