1) selct one for ceter. - nautally, 5
2) devide the rest 8 in two equal group (both have 4 number , and the sum are the same).
note: after 5, can only 1,3,6,8 | 2, 4, 7, 9.
3) place on group to 4 corner and others to 4 middle points.
From 5 | 1,3,6,8 |2,4,7,9 --> 9, 2, 7|4, 5, 6|3,8,1.
From 2) you can only select 1, 3, 5, 7, 9 for cernter. After one, ther result of 2) is unique.
Even you try all 1, 3, 5, 7, 9, it does not take too long.
