Solution attached. Given that the problem puts BEFORE and AFTER in big, bold letters I think the answer provided not completely correct. From some the solutions that I have looked at, it looks like many are adding the transaction amount itself to the Sum BEFORE and Sum AFTER. For the account 2 illustration provided, several submitted solutions were comparing a sum of 10 before/after instead of 6. In most cases this returns the correct answer, except in the case of end points. For the first transaction in any account (Trx 0), there are no DR/CR amounts before it to sum, so I do not see how it meets the required criteria given that the transaction amount itself should not be included in the sum. The same applies to the last transaction in an account where the are no amounts after to sum. In my solution, I do not include the transaction amount in the sums, so for Account 1 the end points (Trx 0 and Trx 7) get filtered out because either the sum BEFORE or the sum AFTER is null.
Not even nearly as clever as @NicoleJohnson's forward & reverse solution.