Describe your change:

I added three indexOf methods to the SinglyLinkedList Java data structure:

indexOf() - finds the logical index of the element specified as the parameter
indexOfFirstAppearance() - finds the logical index of the first appearance of the element specified as the parameter
indexOfLastAppearance() - finds the logical index of the last appearance of the element specified as the parameter

I also added various tests in the driver code to include the implementation of these three methods

• Add an algorithm?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Java files are placed inside an existing directory.
• All filenames are in all uppercase characters with no spaces or dashes.
• All functions and variable names follow Java naming conventions.