Added some useful functions for singly linked list.

[ayush26sharma]

Added three functions in total for singly linked list:

1. reverseList- basically the reverse list from a given position in the list till the end.
   a->b->c->d->e->f
   reverseList(c)
   a->b->f->e->d->c

2. detectLoop - It detects the loop in a given singly linked list using Floyd's tortoise and hare algorithm.
   a->b->c->d->e->f->g->h->c
   linkedlist.detectLoop() gives true in this case.

3. swapNodes - It simply swaps the two given nodes themselves and not just their data. (In place swapping)
   a->b->c->d->e->f
   swapNodes(b,e)
   a->e->c->d->b->f

Describe your change:

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

References

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.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.