Matrix in which most entries are 0.
Arise frequently in engineering applications.
Storing in an array is wasteful.
X = 1 2 3
----------
Y=1 | 0 88 0
Y=2 | 0 0 0
Y=3 | 27 0 0
Y=4 | 19 0 66
multi-linked: each node contains value, (row,column),
next-in-row, next-in-column