Starting position:
0 and -1 denote empty spaces
The empty spaces are always together in upper most position or bottom
left most position.
Solve puzzle by moving spaces to bottom right with the numbers in order.

6 possible directions are symbolized by
 5   0
  \ /
4--*--1
  / \
 3   2
(i.e. 1 = right, 4 = left)

So far all solutions are by David Bagley.