Much simpler version.

for(int i = 0; i < s.length(); i++) { 
    s.at(i) = tolower(s.at(i)); 
} 
sort(s.begin(), s.end()); 
s.erase(unique(s.begin(), s.end()), s.end()); 
s.erase(s.begin());

if(s.length() == 26) 
    cout << "pangram" << endl; 
else 
    cout << "not pangram" << endl;