Moved StackPostfixNotation.java from the Others section to the Stack section

[beradeep]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.

Issue

May solve issue #4360

[vil02]

I am not convinced if an algorithm using a stack should be in the stacks directory. If it should be, then all of the algorithms using any data structure should be in the datastructures directory.

My understanding is that the stacks should contain different implementations of stack.

[beradeep]

I think since this one is closely related to the application of stack, and that there are already algorithms like PostfixToInfix.java inside stacks, so moving StackPostNotation.java into stacks makes sense according to me.

[BamaCharanChhandogi]

I think since this one is closely related to the application of stack, and that there are already algorithms like PostfixToInfix.java inside stacks, so moving StackPostNotation.java into stacks makes sense according to me.

Exactly

[beradeep]

So can it be merged?

[siriak]

Let's create a directory stack in agorithms directory and put stack-related algorithms there. They shouldn't be in datastructures. Besides, please also update file structure of tests to match that of algorithms

[BamaCharanChhandogi]

@beradeep are you working?

[beradeep]

@BamaCharanChhandogi @siriak I have tried to incorporate all the requested changes. Is it okay right now?