public static string ReverseStringPair(string input)

{

    if (string.IsNullOrEmpty(input))

        throw new ArgumentNullException("input");
    char[] result = new char[input.Length];
    int resultIter = 0;
    ReverseStringPairImp(input, 0, result, resultIter);
    return new string(result);
}
private static void ReverseStringPairImp(string input, int inputIter, char[] result, int resultIter)
{
    // skip white space
    while (inputIter < input.Length && input[inputIter] == ' ') inputIter++;
    int[] indics = new int[2] {
        inputIter, // first word begin index,
        0 // second word begin index
    };
    for (; inputIter < input.Length; ++inputIter)
    {
        if (input[inputIter] == ' ')
       {
            if (indics[1] == 0)
                indics[1] = -1;
            else if (indics[1] > 0)
                break;
        }
        else if (input[inputIter] != ' ' && indics[1] == -1)
            indics[1] = inputIter;
    }
    // copy second word, inputIter points to the end of second word
    if (indics[1] > 0)
    {
        for (int i = indics[1]; i < inputIter; ++i)
            result[resultIter++] = input[i];
        indics[1]--;
        // copy white space
        while (indics[1] >= 0 && input[indics[1]] == ' ')
            indics[1]--;
        result[resultIter++] = ' ';
    }
    else
        indics[1] = input.Length - 1;
    // copy the first word
    for (int i = indics[0]; i <= indics[1]; ++i)
       result[resultIter++] = input[i];
    // next pair
    if (inputIter < input.Length)
    {
        result[resultIter++] = ' ';
        ReverseStringPairImp(input, inputIter, result, resultIter);
    }
}

 [TestMethod()]

public void ReverseStringPairTest()

{

    string input = "ab cd ef gh";
    string expected = "cd ab gh ef";
    string actual = Program.ReverseStringPair(input);
    Assert.AreEqual(expected, actual);
}