I'm looking at an example of an eulerian path problem, and it's not clear to me what the problem is.
There are N dominoes, as it is known, on both ends of the Domino one number is written(usually from 1 to 6, but in our case it is not important). You want to put all the dominoes in a row so that the numbers on any two adjacent dominoes, written on their common side, coincide. Dominoes are allowed to turn.
What does it mean for "numbers on any 2 adjacent dominoes, written on their common side, coincide?"
Are the dominoes laying flat, like this:
Or are the dominoes standing up? That part wasn't clear. If it's the latter, then that makes more sense. If it's the former, then the question seems moot.
This is a question from https://cp-algorithms.com/graph/euler_path.html#toc-tgt-1