a[n][n + 1] be the two-dimensional array used
to store the adjacency matrix
A of an n vertex graph.
The diagonal entries of
A are not stored in
a. However, all diagonal elements
of A are required to be zero.
Elements A(i,j) with i < j
are in the upper triangle of A and are stored
in a[i-1][j-1].
Elements A(i,j) with i > j
are in the lower triangle of A and are stored
in a[i-2][j-1].