USING SET(C++) to check if two given strings are anagrams in linear time

Two strings str1 and str2 are of equal length.
I used unordered_map to solve this problem by counting the frequency of each character in the word which works just fine BUT
I want to use unordered_multiset to solve this problem but I am getting run-time error.
I have explained my code with comments, please follow the link

https://ideone.com/IQUaTk

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