实现简单的LZW压缩算法，LZW压缩算法的基本概念：LZW压缩有三个重要的对象：数据流(CharStream)、编码流(CodeStream)和编译表(String Table)。在编码时，数据流是输入对象（文本文件的据序列），编码流就是输出对象（经过压缩运算的编码数据）；在解码时，编码流则是输入对象，数据流是输出对象；而编译表是在编码和解码时都须要用借助的对象。字符(Character)：最基础的数据元素，在文本文件中就是一个字节；字符串(String)：由几个连续的字符组成； 前缀(Prefix)：也是一个字符串，不过通常用在另一个字符的前面，而且它的长度可以为0；

-Achieve a simple LZW compression algorithm, LZW compression algorithm' s basic concept: LZW compression three important objects: data stream (CharStream) encoded stream (CodeStream) and compile Table (String Table). When coding, the data stream is input to the data sequence of objects (text file), encoded stream output object (after compression operation code data)　When decoded, the encoded stream is input object, the data stream is output objects　while compiling The table is a need to use a means in the encoding and decoding are objects. Character (Character): The most basic data elements, in the text file is a byte　string (String): composition by several successive characters　prefix (prefix): is a string, but usually used in another in front of the character, and its length can be 0