Hi all, at http://en.wikipedia.org/wiki/Knuth's_Algorithm_X in the example I don't understand that why not "A" is in the final solution & only B, D & F are in the final solution whereas "A" was selected at Level 1
plus why at the level 1 the branch containing "D" (i.e. containing 0, 1, 1, 1 in columns 2, 3, 5 & 6) was terminated unsuccessfully?
The intent, as far as I understand it, is to find a combination of rows that include every number from 1 to 7 but where each number only occurs in at most one row.
The algorithm seems to work by picking the row (A) that has the most numbers in it and then trying to find a combination of the other rows that have the numbers that aren't in A.
Having selected A (in this case), it proceeds by removing any sets that have a number in common with A (in this case, B,C,E and F). This is because any combination of B,C,E or F with A will have at least one multiple of '1', '4' or '7'. This only leaves row D as a possible match for A.
However, row D doesn't contain a '2' and neither does A, so the combination of A and D is not an exact cover, hence the algorithm terminates and goes to test for another combination.