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SUBMIT YOUR IDEAHi,
I created the solutions, but my results are slightly different. the reason is easy: According to Use Case we should understand correlation of merchants, so we need also information about those customers who visit only one merchant. However, I do not understand why we should keep lines with nulls where counts are between 2 and 4.
That's why I my solutions I exclude such situations (which is easy to add).
Solution 2:
What a weird challenge, found an easy way to do it apart from the (useless imo) nulls, so left it there.