+a1
+a2
...
+a5000
?a
?a
?a

and 5000 queries ?a
or this operations in random order
IMHO O(N^2) - is best. I am right?

I try to solve this with red&blak tries but WA#5
what's in this test?

I had WA#5 when my program gives more then 20 names of stars.