Data Mining 2011

Today, I got results from Data Mining Cup 2011. In results I occured on 15 place (from 35) for first task, and doesn't occured in second's task table at all (but I want to know why).

Still my results wasn't so bad for that simple algorithm (on this later) I made:

Number	Name	Score	Comment
1.	TU_Dortmund_1	69835
...	...	...
12.	Uni_Siberian_Telecommunication_1	60550
13.	TU_Wien_1	51704	<-- strange jump in scores
...	...	...
15.	Uni_Kharkov_1	50627	<-- me
...	...	...
30.	Uni_Chile_2	21018
...	...	...
35.	Inst_Telkom_2	1230

So, as you see, I occured on middle group, and as I think, in this group we all used pretty same alogrithm - modifications of nearest neighborhood. But top 12 people have another alogithm, and I'll try to figure out what it was.
Still now I am describing mine algorithm.
For learn step I made hash map, where key was item number and value - hash maps, which contains as keys - items that was viewed\ordered in the same session of initial key, and value - count of views\orders.
For example:

1000|1|0
1000|2|0
1000|3|0
1001|1|0
1001|2|0

And in result I'll have {'1': {'2': 2, '3': 1}, '2': {'1': 2, '3': 1}, '3': {'1': 1, '2': '1'}}.
And when I needed to get test results, I for each session, gather items that was already viewed\ordered, and merge values of this hash map. Than sort values and return top 3 of them, which wasn't already in session.
For example:

1003|1|0
1003|2|0

Merged hashmap will be: {'1': 2, '2':  2, '3': 2, '4': 1, '5': 3}. Next removing 1 and 2, because they already in current session: {'3': 2, '4': 1, '5': 3}. Sort by value and return keys: [5, 3, 4].
But if there aren't 3 values in resulting list, I will additionaly return top selling items. For example, after sort step list was: [3], than adding top sellers it will be: [3, 10, 15], where 10 and 15 - top 2 selling items, which were calculated on learn step.
Additionally, I used weight for views\add to cart\orders - 1, 5, 10 when building learning hashmap.

For task 2 I made the same, but it builds this hash map online and returns current best choices on each step.
But I'm very interested how top1-3 teams solved this tasks. If I'll find out and they'll allow - I'll post it here =)