I have been trying to solve this problem but I do not how to proceed.

I tried two different approaches:

#### First...

Store all possible combinations of rows that sum the magic number. Then solve the problem using backtrack: 'size' nested for loops and checking the constrains (sum(cols), sum(rows) and sum(diagonals)) each step. Of course with as much optimizations as I realized. With size of 6, the first set of combinations is 6000 long, and I got this number reduced to 1000 after third loop. (something like 6000 combs first loop, 4000 second, 6000 third, 1000 fourth, 6000 fifth, 500 sixth)

#### Second...

Store all domino pieces and again used backtracking to solve the problem. This case recursively adding a piece each step, checking conditions, if pass all conditions keep adding pieces, else try another piece until last piece (in this case go back and modify the previous piece).

==================

For size = 4 it is okey. It does not take long and solves everything. But with size = 6 it takes so many minutes.

Can anyone help me? I really want to solve this problem but I need a clue or something :/

Created at: Feb. 1, 2017, 6:23 a.m.; Updated at: Feb. 4, 2017, 7:24 p.m.