CleanChat

PROBLEM STATEMENT

You're building a "clean" chat room, so you want to be able to filter out
certain words. Given a "proposed" text message that a user has typed and a list
of banned words, return a filtered version of the message in which any banned
words have been replaced by asterisks (*).

DEFINITION
Class: CleanChat
Method name: filterWords
Parameters: String, String[]
Return type: String

Method signature:  String filterWords(String message, String[] banned); (be
sure your method is public)

NOTES
- Each banned word is replaced by a series of asterisks, one asterisk for each
letter in the word.
- Banned words should be recognized in a case-insensitive way.
- Banned words will be blocked even if they appear within a longer word (see
examples).
- Banned words may overlap. Letters may therefore be replaced by asterisks
multiple times, but the result will be the same as if they were replaced once
(see examples).

TopCoder will ensure that:
* The message may contain only upper and lower case letters [A-Z,a-z], digits
[0-9] (inclusive), and some punctuation: period, exclamation mark, comma,
question mark, semicolon, colon, and parentheses: .!,?;:()
* The message will be 0 to 50 characters in length, inclusive.
* Each banned word will be 1 to 20 characters in length, inclusive.
* Each banned word will contain only lower case letters [a-z], no punctuation,
numbers or spaces.
* There will be 0 to 50 banned words, inclusive.

EXAMPLES (Quotes are for clarity only.)

message = "Testing the filter."
banned = ["test","sting","he"]
First, the Test in Testing is filtered out. Next, "sting" is replaced; even
though the "st" was already replaced in the first step, "sting" still must be
recognized and replaced because it appeared in the original string. Finally,
"he" is filtered out, and the end result is:
Return value: "******* t** filter."

message = "Thou testest the filter."
banned = ["test"]
Return value: "Thou ******* the filter."

message = "Testing with no filter."
banned = []
Return value: "Testing with no filter."

message = "No,words split by punctuation are not blocked."
banned = ["now"]
Return value: "No,words split by punctuation are not blocked."

message = "Nor are words split by a space."
banned = ["rare"]
Return value: "Nor are words split by a space."

message = "Punctuation? Why, yes; there is! (Of course. :)"
banned = ["on","y","is","ours","of"]
Return value: "Punctuati**? Wh*, *es; there **! (** c****e. :)"

message = ""
banned = ["empty","message"]
Return value: ""