vector<int> gradingStudents(vector<int> grades) {
    // grades already passed by value
    std::transform(grades.begin(), grades.end(), grades.begin(), 
        [] (int grade) {
            if (grade < 38)
                return grade;
            
            int toNextMultiple = 5 - grade % 5;
            
            if (toNextMultiple < 3) {
                grade += toNextMultiple;
            }
            
            return grade;      
        });
        
    return grades;
}