My IsIsogram

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1 
bool FBullCowGame::IsIsogram(FString Word) const
{
    // treat 0 ans 1 letter words as isograms
    if (Word.length() <= 1) { return true; }
    TMap<char, bool> LetterSeen; // setup our map
    for (auto Letter : Word)    // for all letters of the word
    {
        Letter = tolower(Letter); // handle mixed case
        if (LetterSeen[Letter])   // if the letter is in the map
        {
            return false; // we don't have an isogram
        }
        else // otherwise
        {
            LetterSeen[Letter] = true; // add the letter to the map as seen
        }            
    }
    return true;
}
2

Looks good to me.