DetectCapitalTheSolution.cs

public bool DetectCapitalUse(string word)
{
    if (word.Length == 1) return true;
    if (word[0] <= 'Z' && word[1] <= 'Z')
    {
        for (var i = 2; i < word.Length; i++)
        {
            if (word[i] <= 'Z') continue;
            return false;
        }
    }
    else
    {
        if (word[1] <= 'Z') return false;

        for (var i = 2; i < word.Length; i++)
        {
            if (word[i] >= 'a') continue;
            return false;
        }
    }

    return true;
}