Arrays and Iteration

Two-Sum with Hash Lookup

Walk the array once, storing seen values in a lookup table. When the complement is already present, the result indices are known.

Algorithm

execution replay The checked-in replay follows the language-neutral state table for `array-two-sum-hash`.
cross-language comparison This Dart DSA version keeps the same data and final output as every other DSA book in this wave.

Basic Implementation

basic.dart
Replay: real traced execution (multi-file project)
void main() {
  final arr = [2, 7, 11, 4, 5];
  const target = 9;
  final seen = <int, int>{};
  var first = -1;
  var second = -1;
  for (var i = 0; i < arr.length; i++) {
    final value = arr[i];
    final need = target - value;
    if (seen.containsKey(need)) {
      first = seen[need]!;
      second = i;
      break;
    }
    seen[value] = i;
  }
  print('[$first, $second]');
}

  1. arr ← [2, 7, 11, 4, 5], target ← 9, seen ← {}

    1void main() {2  final arr = [2, 7, 11, 4, 5];
    values this step[2, 7, 11, 4, 5]arr9target{}seen

  2. seen ← {2: 0}, hit ← no

    8final value = arr[i];9final need = target - value;10if (seen.containsKey(need)) {
    values this step{} → {2: 0}seennohit0i2arr[i]7need

  3. hit ← yes, result ← [0, 1]

    8final value = arr[i];9final need = target - value;10if (seen.containsKey(need)) {
    values this stepyeshit[0, 1]result1i7arr[i]2need{2: 0}seen

Complexity

  • Time: O(n) average
  • Space: O(n)

Implementation notes

  • Keep the explicit control flow. Library shortcuts would hide the state changes this lesson is meant to replay.
  • The final output is intentionally small and deterministic for cross-language comparison.