coordinate representation

$A$ can become a set of $|E|$ tuples of the form $(\text{row index},\text{column index},\text{value})$. So this matrix becomes the ordered set of these tuples, sorted by row and then column index.

This can be better from a memory perspective, but can still be quite large if we have a few nodes with high degree. A nice solution to this is to use compressed sparse row representation instead.

$$A=\left[\begin{array}{cccc}12& 0& 26& 0\\ 0& 0& 19& 14\\ 26& 19& 0& 0\\ 0& 14& 0& 7\end{array}\right]$$

can become
$(0,0,12),(0,2,26),(1,2,19),(1,3,14),(2,0,26),(2,1,19),(3,1,14),(3,3,7)$