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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.
yeah.. abc123 has each character - a,b,c,1,2,3 only once.
aabb has each letter twice - a:2 , b:2.
So, each character has to be in multiples of same numbers
With 13, your if condition isn't true and you return an empty array instead.
Read the description a couple more times, all you need is there.