count the frequency of each character in a text.

// Challenge: count the frequency of each character in a text.
// Input: "banana"
// Expected output: {b: 1, a: 3, n: 2}

// Solution: we walk through the text and use a Map to keep the count.
// The ?? operator and update help us initialize at 0 the first time.
Map<String, int> countCharacters(String text) {
  final counts = <String, int>{};
  for (final character in text.split('')) {
    // If the character was already there, add 1; otherwise, start at 1.
    counts[character] = (counts[character] ?? 0) + 1;
  }
  return counts;
}

void main() {
  print(countCharacters('banana')); // {b: 1, a: 3, n: 2}
  print(countCharacters('aabbc')); // {a: 2, b: 2, c: 1}
  print(countCharacters('')); // {} (edge case: empty text)

  // Explanation:
  // We walk through each character. The key to the trick is (counts[character] ?? 0):
  // if the key does not exist yet, counts[character] is null, and with ?? 0
  // we start at 0 before adding 1. That way we avoid a null error.
}