Efficient way to find Frequency of a character in a String in java : O(n)?

I'd use an array rather than a hash map. If we're limited to ascii, that's just 256 entries; if we're using Unicode, 64k. Either way not an impossible size.

Besides that, I don't see how you could improve on your approach. I'm trying to think of some clever trick to make it more efficient but I can't come up with any.

I think your approach is in theory the most efficient (O(n)). However in practice it needs quite a lot of memory, and is probably very slow.

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