Merge pull request #1 from cnishina/master

convert to maven project
Merge pull request #3 from cnishina/master

fix parens validation if they match
Merge pull request #4 from cnishina/master

removing extra loop to match open and closed symbols
adding multiple pop test to stack/queue
queue pop method, tail should set the next node to null
Merge pull request #5 from cnishina/master

Doubly linked list
bstree to a single linked list (eXtreme programming with Evan)
Merge branch 'master' of https://github.com/WhiteBoardDev/java-datastructures