使用两个数组完成

    byte index = 0;
    char[] str = new char[128];
    byte[] count = new byte[128];

    //Insert one char from stringstream
    public void Insert(char ch)
    {
        str[index++] = ch;
        count[ch]++;
    }
  //return the first appearence once char in current stringstream
    public char FirstAppearingOnce()
    {
        for (int i = 0; i < str.length; i++) {
            char c = str[i];
            if (count[c] == 1) {
                return c;
            }
        }
        return '#';
    }