Adding C++ Solutions to LeetCode #2 and #3

[williamgtrigg]

Pull Request Template

Description

I am attempting to push the solutions to two LeetCode Problems entitled "Add Two Numbers" and "Longest Substring Without Repeating Characters". These are both Medium difficulty problems that I solved using C++.

Put check marks:

Have you made changes in README file ?

• [x] Added problem & solution under correct topic.
• [x] Specified Space & Time complexity.
• [x] Specified difficulty level, tag & Note(if any).

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also note any relevant details for your test configuration.

• [x] Test A: I tested the functionality of my two solutions on my own computer with extensive test cases, including any edge cases that I could think of.
• [x] Test B: I confirmed the functionality of my two solutions by getting them to pass all test cases on LeetCode.

Make sure all below guidelines are followed else PR will get Reject:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code so that it is easy to understand
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published in downstream modules

[williamgtrigg]

Is there anything else I need to do to merge this PR?