Hash Tables

Group by Key

Build buckets keyed by a shared field, preserving the first-seen key order.

Algorithm

Canonical pairs (a,1), (b,2), (a,3), (c,4), (b,5) print {a: [1, 3], b: [2, 5], c: [4]}. The replay uses the same input in every language, so this Dart DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.dart 
void main() {
  final pairs = [('a', 1), ('b', 2), ('a', 3), ('c', 4), ('b', 5)];
  final groups = <String, List<int>>{};
  for (final pair in pairs) {
    groups.putIfAbsent(pair.$1, () => <int>[]).add(pair.$2);
  }
  final parts = groups.entries.map((entry) => '${entry.key}: [${entry.value.join(", ")}]');
  print('{${parts.join(", ")}}');
}

Complexity

  • Time: O(n) average
  • Space: O(k + n) for buckets and values

Implementation notes

  • Keep output formatting deterministic. Do not rely on unordered hash-map printing when the lesson needs cross-language comparison.
  • The trace highlights the hash table state after each write.
bucket map Each key owns a list. A new key creates a bucket; a repeated key appends to the existing bucket.