Clean attempt at keyboard row https://pastebin.com/RQbfEqWZ

class Solution {

    class KeyboardRow {
    public:
        bool exists_in_single_row(std::string str){
            std::transform(str.begin(), str.end(), str.begin(), [](char s){ return std::toupper(s); });
            bool r1 = std::all_of(str.begin(), str.end(), [this](char c){ return in_row1(c); });
            bool r2 = std::all_of(str.begin(), str.end(), [this](char c){ return in_row2(c); });
            bool r3 = std::all_of(str.begin(), str.end(), [this](char c){ return in_row3(c); });
            return r1 || r2 || r3;
        }

    private:

        bool in_row1(char c)
        { return std::find(std::begin(row1), std::end(row1), c) != std::end(row1); }

        bool in_row2(char c)
        { return std::find(std::begin(row2), std::end(row2), c) != std::end(row2); }

        bool in_row3(char c)
        { return std::find(std::begin(row3), std::end(row3), c) != std::end(row3); }

    private:
        const char row1[10] = {'Q','W','E','R','T','Y','U','I','O','P'};
        const char row2[9] = {'A','S','D','F','G','H','J','K','L'};
        const char row3[7] = {'Z','X','C','V','B','N','M'};
    };

public:
    vector<string> findWords(vector<string>& words) {
        KeyboardRow kr;
        std::vector<std::string> result;

        std::copy_if(words.begin(), words.end(), std::back_inserter(result),
                    [&kr](const std::string& s){ return kr.exists_in_single_row(s); });
        return result;
    }
};