If you've got WA #11 then your program deals bad with repeatative queries.

Try this test:
--------------------
2
acm 10000
acmicpc 10000
2
ac
ac
---------------------
Correct answer is:
---------------------
acm
acmicpc

acm
acmicpc
---------------------

this code works fine for above input but still gives wa#11

I believe, this will help you

18
a 10
ba 0
bb 1
bc 1
bd 1
be 1
bf 1
bg 1
bh 1
bi 1
bj 1
bk 1
bl 1
bm 1
bn 1
bo 1
bp 1
bq 1
1
b

ba can't be with zero frequency: 1 ≤ n_i ≤ 106