I have used hash_map for some problems on Valladolid - it works. Judge is saying he does not know what 'collate' is... As far as I understand you are using collate for hashing. You have two choices:
1) Figure out how to make the judge understand collate.
2) Implement you own hashing. (shifting, bitwise ^&|, no -+*/%)

In any case I recommend you to switch string to char[] - strings are twice as slow as char[].

Ask you if have any specific questions.