Renamed files and added 5 problems

[zahyaah]

Added following Leetcode solutions

Issue #78

1. 1351 Count Negative Numbers in a Sorted Matrix

2. 1572 Matrix Diagonal Sum

3. 231 Power of Two

4. 43 Multiply Strings

5. 1346 Check If N and Its Double Exist

Renamed the files and attached links inside them.

[Adarsh-jaiss]

• Looks good to me 😄

[zahyaah]

Thank you @Adarsh-jaiss It's my first Hacktoberfest contribution :)

[Adarsh-jaiss]

• Please change the directory and create the new PR, I need to revert it because you have added it in main directory other than adding it to the folder

[zahyaah]

So, I need to create a new branch and submit it to some other branch of codemistic/Data-Structures-and-Algorithms rather than main, right? @Adarsh-jaiss