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This solution has O(String.length s) space complexity because String.lowercase_ascii creates a new string.

Thanks for the pointer (no pun intended)! In hindsight that's a clear optimisation.

~- is the integer negation operator.

@ concatenates two lists. Here is the official reference.

no no no. react key you should use when you map some array/list. separate component dont know nothing about react key.

Thanks for your help.