As you are counting each character no matter if it is on the left or right side, you can use just one result structure. Datatype Set is nice if you just want to know the different kind of characters that occures but you also want the count so I suggest to change your result data type to a Map so you can keep a counter for every character.
So replace leftSide and rightSide by a single Map instance and for every character you encounter, check if it exists already in your Map. If so, add one to the value part of that entry, if not put the new character to the map with value 1.
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