Added slow and fast pointers algorithms #14

[aakhtar3]

Welcome to JavaScript community

Describe your change:

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

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 JavaScript files are placed inside an existing directory.
• All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames.
  Example:UserProfile.js is allowed but userprofile.js,Userprofile.js,user-Profile.js,userProfile.js are not
• 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}.

[lgtm-com]

This pull request introduces 4 alerts when merging 9474afb into c419ff5 - view on LGTM.com

new alerts:

• 3 for Missing variable declaration
• 1 for Comparison between inconvertible types

[lgtm-com]

This pull request introduces 1 alert when merging acafd46 into e92b5d2 - view on LGTM.com

new alerts:

• 1 for Comparison between inconvertible types

[lgtm-com]

This pull request introduces 1 alert when merging c0a4353 into e92b5d2 - view on LGTM.com

new alerts:

• 1 for Comparison between inconvertible types