Problem 1812. Tridiagonal

Return an n-by-n matrix that has a, b, c as the subdiagonal, main diagonal, and superdiagonal entries in the matrix.

Example

 a=1
 b=2
 c=3
 n=5
 T =  [2     3     0     0     0;
       1     2     3     0     0;
       0     1     2     3     0;
       0     0     1     2     3;
       0     0     0     1     2]