Binary Search First Occurrence

Find the first copy of a duplicated target by recording matches and continuing to search the left half.

Algorithm

Basic Implementation

basic.dart

void main() {
  final arr = [1, 2, 4, 4, 4, 7, 9];
  const target = 4;
  var lo = 0;
  var hi = arr.length - 1;
  var result = -1;
  while (lo <= hi) {
    final mid = lo + (hi - lo) ~/ 2;
    if (arr[mid] == target) {
      result = mid;
      hi = mid - 1;
    } else if (arr[mid] < target) {
      lo = mid + 1;
    } else {
      hi = mid - 1;
    }
  }
  print(result);
}

Complexity

Time: O(log n)
Space: O(1)

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.

execution replay The checked-in replay follows the language-neutral state table for `search-binary-first`.

cross-language comparison This Dart DSA version keeps the same data and final output as every other DSA book in this wave.