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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
After reviewing the task description, I understand your concerns, but please note that the first line of the task clearly states: "Take 2 strings s1 and s2 including only letters from a to z". Therefore, we can assume that the input will only contain lowercase letters from 'a' to 'z' and we do not need to check for other characters.