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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.
For this problem, the first string is the input string and the second string is represented by the suffixes of the first string, subsequently.
In explanation it says "for second string", what second string is he referring to ?