fix for quadratic time caused by substring() in LZW compression

[inflaton]

Hi Kevin,

I've made a fix for quadratic time caused by substring() in LZW compression. The key to the fix is to use a new overloaded method TST.longestPrefixOf(String query, int startIndex) in LZW.compress() method. Attached please find 3 reference files:

1. LempelZivWelch.java: improved LZW.java

2. TernarySearchTrie: improved TST.java

3. lzw.sh: shell script proves the correctness and performance improvement.
   NOTE: existing LZW seems to take forever to compress dickens.txt (~29MB)

Regards,
Donghao

LZW_fix.zip