consists of two half-Dominoes. Each half-domino shows a number between 0 and
- ➋ Each domino exists exactly once: 0-0, 0-1, etc. up to N-N.
Note that there is no difference between X-Y and X-Y (e.g. 1-2 and 2-1).
Divide the diagram along the grid lines into dominoes so that each domino
occurs exactly once.