Add WelshPowell (Graph Colouring)

[straf10]

• 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.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[straf10]

i try to format based on clang format but i cant. Dowloaded llvm added to enviroment variables and path and then my computer finds the clang format. After that i dont know what to do.

[vil02]

i try to format based on clang format but i cant. Dowloaded llvm added to enviroment variables and path and then my computer finds the clang format. After that i dont know what to do.

You can have a look at the log and introduce the required changes manually.

If you are having trouble with running clang-format, have a look here: #4438 (comment).

[vil02]

This is just the first iteration.

[straf10]

i think i changed everything you asked, please inform me if i need to change or fix anything else.
Thanks in advance.

[vil02]

Regarding the general design of this class, I would suggest the following (pseudo code):

class WelshPowell {
    private static class WPGraph {
        // almost same as now
    }

    public static WPGraph makeGraph(int numberOfVertices, listOfEdges) {
        // here you create the graph
    }

    public static int[] welshPowellColoring(WPGraph graph) {
        // as it is now
    }
}

Then, you could write the first test like (still pseudo code)

    final var graph = WelshPowell.makeGraph(3, {{0, 1}, {1, 2}, {2, 0}});
    final var colors = WelshPowell.welshPowellColoring(graph);
    // check if the colors is as expected
    assert(...);

Thanks to this, you will be able to get rid of some methods.

[vil02]

We are almost there! Let's sped some time to make everything perfect.

[straf10]

i think i resolved almost all of the changes requested. Have a look and let me know if anything else needs to be changed.

[vil02]

Please add the missing tests. By the way, in same cases you can use Commit suggestion button. It is quite handy.

[vil02]

Almost done!

[straf10]

i added the complex test based on the website, add the blank_color method, updated the comments on the precolored test, and followed all your suggestions. Have a look for any minor changes needed but all in all I think it is ready.

[vil02]

Good work!

For a future project you can consider adding a method trying to determine if the returned coloring is optimal.