create a hash to hold processed pairs, with a key like "3 4" to 
indicate that 3 has been processed against 4

get minimum and maximum key values (3 & 11 in this case)
loop from minimum to maximum minus interval
    if interval contains more than one item
        for each pair of items in interval
            check hash to see if the pair has been processed
            if not
                 process the pair
                 mark the pair in the hash