Create uniqueBinarySearchTree.java

Count UniqueBST using dp catalan number approach.

Describe your change:

• [ ] Add an algorithm?
• [ ] Fix a bug or typo in an existing algorithm?
• [x] Documentation change?

References

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [x] All new Java files are placed inside an existing directory.
• [x] All filenames are in all uppercase characters with no spaces or dashes.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.