DetectCapitalDesignTime.cs

public bool DetectCapitalUse(string word)
{
    var n = word.Length;
    bool match1 = true, match2 = true, match3 = true;

    // case 1: All capital
    for (int i = 0; i < n; i++)
    {
        if (!char.IsUpper(word[i]))
        {
            match1 = false;
            break;
        }
    }

    if (match1)
    {
        return true;
    }

    // case 2: All not capital
    for (int i = 0; i < n; i++)
    {
        if (!char.IsLower(word[i]))
        {
            match2 = false;
            break;
        }
    }

    if (match2)
    {
        return true;
    }

    // case 3: All not capital except first
    if (!char.IsUpper(word[0]))
    {
        match3 = false;
    }

    if (match3)
    {
        for (int i = 1; i < n; i++)
        {
            if (!char.IsLower(word[i]))
            {
                match3 = false;
                break;
            }
        }
    }

    if (match3)
    {
        return true;
    }

    // if not matching
    return false;
}