Warning, spoiler?

My first take on it was the following: Take the average of the differences between each pixel and the pixel adjacent to it over each column and store that as, say, X[]. Find some maximum number such that the sum of every nth column of X - the sum of all other rows of X is maximised. That's the column width. Split into a series of columns, use stable matching to match columns based on the sum of differences between rightmost male pixel and leftmost female pixel over all rows. Use stable marrage to give you a partial ordering, turn into a complete ordering and unscramble the image.

Any better/more elegant solutions?