Why my code breaks when I put pure.empty() as 'else if' after the first 'if' condition, but works when it's positioned as first 'if'? [closed]

My all test cases passed except when input is only dots"...", I want it to print "Valid".

It worked with code below:

int main()
{
        int L;
        cin >> L;
        vector<char> vec(L);
        //input
        for (int i = 0; i < L; i++)
            cin >> vec[i];
        //removing dots
        vector<char> pure; 
        for (int i = 0; i < L; i++)
        {
            if (vec[i] == 'H' || vec[i] == 'T')
                pure.push_back(vec[i]);
        }
        
        int pureSize = pure.size() - 1;
         //     HERE
        if (pure.empty())
            cout << "Valid" << "\n";
        else if (pure[0] == 'H' && pure[pureSize] == 'T')
        {
            int flag = 1;
            for (int i = 0; i < pureSize - 1; i++)
            {
                if (pure[i] == pure[i + 1])
                {
                    flag = 0;
                    break;
                }
            }
            if (flag)
                cout << "Valid"
                     << "\n";
            else
                cout << "Invalid"
                     << "\n";
        }
        else
            cout << "Invalid"
                 << "\n";
    return 0;
}

BUT why this doesn't work/ breaks (below) when I interchange the positions of first two conditions? It should never enter in first 'if' if I enter only dots'...'?

int main()
{
        int L;
        cin >> L;
        vector<char> vec(L);
        //input
        for (int i = 0; i < L; i++)
            cin >> vec[i];
        //removing dots
        vector<char> pure; 
        for (int i = 0; i < L; i++)
        {
            if (vec[i] == 'H' || vec[i] == 'T')
                pure.push_back(vec[i]);
        }
        //logic
        int pureSize = pure.size() - 1;
        //      HERE
        if (pure[0] == 'H' && pure[pureSize] == 'T')
        {
            int flag = 1;
            for (int i = 0; i < pureSize - 1; i++)
            {
                if (pure[i] == pure[i + 1])
                {
                    flag = 0;
                    break;
                }
            }
            if (flag)
                cout << "Valid"
                     << "\n";
            else
                cout << "Invalid"
                     << "\n";
        }
        else if (pure.empty())
            cout << "Valid" << "\n";
        else
            cout << "Invalid"
                 << "\n";
    return 0;
}

Why the first code works but second doesn't? I mean in the second code it should jump to else if since first condition is never met, right? Since "..." is the input, there is no H or T. Please explain.