char firstNotRepeatingCharacter(std::string s)
{
    for(int searchedChar = 0; searchedChar < s.size();)
    {
        std::vector <int> duplicatePositions;

        for(int possibleDuplicate = searchedChar+1; possibleDuplicate < s.size(); possibleDuplicate++)
        {
            if(s.at(searchedChar) == s.at(possibleDuplicate))
            {
                duplicatePositions.push_back(possibleDuplicate);
            }
        }

        if(duplicatePositions.size() > 0)
        {
            for(int i = 0; i < duplicatePositions.size(); i++)
            {
                s.erase(s.begin()+(duplicatePositions.at(i)-i));
            }

            s.erase(s.begin()+searchedChar);
        }
        else
        {
            return s.at(searchedChar);
        }
    }

    return '_';
}


/////////////////////////////////////////////////////////////////////////////////////////////////////////

char firstNotRepeatingCharacter(string s) {
    int count[26] = {};
    int order[26];
    int i = 0;
    for (c : s)
        if (!count[c-'a']++)
            order[i++] = c-'a';
    for (int j = 0; j < i; ++j)
        if (count[order[j]] == 1)
            return order[j]+'a';
    return '_';
}