defaultdict(int) for counting

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defaultdict(int) uses int as the factory, so accessing a missing key creates it with 0 (the result of int()). On the first occurrence of 'a', counts['a'] is auto-initialized to 0, then += 1 makes it 1. The second 'a' increments it to 2. This pattern replaces the more verbose 'counts[ch] = counts.get(ch, 0) + 1'.