void Main()
{
    string input = "sports.sina.com.cn"; 
    char[] array = input.ToCharArray();
    
    string output = ReversePartial(input.ToCharArray(), '.');
    Console.WriteLine(output);
}


string ReversePartial(char[] input, char seperator)
{
    Reverse(ref input, 0, input.Length - 1);
        
    char tmp = new char();
    int head, tail;
    head = tail = 0;
    
    for(int i = tail; i < input.Length; i++)
    {
        if (input[i] == seperator)
        {
            tail = i - 1;
            
            if (head < tail)
            {
                Reverse(ref input, head, tail);
                    head = i + 1;                
            }    
        }
    }
    if (head > tail && head < input.Length)
        Reverse(ref input, head, input.Length - 1);
    
    return new string(input);
}
void Reverse (ref char[] input, int startIndex, int endIndex)
{
    char tmp = new char();
    for(int i = 0; i <= (endIndex - startIndex) / 2; i++)
    {
        tmp = input[startIndex + i];
        input[startIndex + i] = input[endIndex -i];
        input[endIndex -i] = tmp;
    }
}