Added three more approaches in anagram.java

[Anshumanshukla46]

First approach (5): Here, I have used a HashMap and once the element is found in s frequency have increased by 1 and when found in frequency have decreased by 1.
Second approach (6): Here, I have used an array of size 26 where I increase the counter by 1 if found in s and decreased by 1 if found in t. If somehow it went in negative, I simply return false.
Third approach (7): Here, I have used an array of size 256 which have the ability to deal with all Unicode characters.

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