Given a list of account transactions such as A:+100, B:+200, A:-50, C:+300, B:-100, A:+150, calculate the final balance for each account — explain approach, write code, time/space complexity, and solve using Java 8 features like Map.merge()

Tests whether you reach for Map.merge(account, amount, Integer::sum) to fold transactions into running balances in a single O(n) pass, rather than grouping transactions first and summing each group separately in a second pass.