Arrays and Iteration

Reverse Array In Place (Two Pointers)

Walk two indices toward each other from the ends of the array, swapping at each step. The two-pointer pattern with the smallest possible state. The loop stops when the indices meet or cross.

Algorithm

The canonical input [1, 2, 3, 4, 5, 6, 7] reverses to [7, 6, 5, 4, 3, 2, 1] after three swaps. The middle element at index 3 is untouched because the pointers meet there.

Basic Implementation

basic.dart 
void main() {
  final arr = <int>[1, 2, 3, 4, 5, 6, 7];
  var left = 0;
  var right = arr.length - 1;
  while (left < right) {
    final tmp = arr[left];
    arr[left] = arr[right];
    arr[right] = tmp;
    left = left + 1;
    right = right - 1;
  }
  print(arr);
}

Complexity

  • Time: O(n)
  • Space: O(1)

Implementation notes

  • Dart: use the explicit tmp = arr[left]; arr[left] = arr[right]; arr[right] = tmp; triple. Avoid arr.reversed.toList(), List.from(arr.reversed), or assigning to arr from a reversed view; all hide the step-by-step pointer walk the lesson is teaching.
  • Replay highlights both left and right per frame plus the new array contents after each swap, matching the lesson spec.
two pointers Indices walk toward each other and swap.