Left/Right arrow to navigate through posts in the same category only

Hello all,

I have a quick question on how to group posts with the same category together!

Here is the site I need help on: http://www.lvtran.com/project-euler/

I currently have 4 posts, 3 of them are categorized as “euler” and one of them is not. As you can see in the site I provided, I managed to group the 3 euler-categorized posts together. However, when I click on one of the 3 posts and use the left and right arrow to navigate through all the posts, I still see the post called “Post 1 Test”, which is NOT categorized as “euler”.

Is there a way so that the left and right arrow only navigate through a list of posts that are in the same category?

Thank you so much for your help!