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I'm afraid you're wrong. Brute force solution runs within 0.78 sec, and it takes about 10 minutes to write it from scratch.The idea is that you don't need 0, and need 1-9, so that sum matches 45. Than you over-force problem for solution. As an easy corollary you understand that in center must be five, and other 4 pairs will than sum to 10 each.