Reverse Array In Place (Two Pointers)

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

Algorithm

Canonical input (1, 2, 3, 4, 5, 6, 7) (odd length, middle element stays put) yields three swap frames and reverses to [7, 6, 5, 4, 3, 2, 1].

Basic Implementation

basic.pl

use strict; use warnings;
my @arr = (1, 2, 3, 4, 5, 6, 7);
my $left = 0;
my $right = scalar(@arr) - 1;
while ($left < $right) {
	my $tmp = $arr[$left];
	$arr[$left] = $arr[$right];
	$arr[$right] = $tmp;
	$left = $left + 1;
	$right = $right - 1;
}
print "[" . join(", ", @arr) . "]\n";

Complexity

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

Implementation notes

Perl: explicit three-line my $tmp = $arr[$left]; $arr[$left] = $arr[$right]; $arr[$right] = $tmp swap keeps the move visible. The stdlib reverse @arr would hide the lesson entirely (and returns a new list rather than mutating in place), and @arr[$left, $right] = @arr[$right, $left] (list-slice assignment) would collapse the swap into a single frame.
$left = 0 and $right = scalar(@arr) - 1 use plain integer indices; the $left < $right guard handles the meet-in-the-middle exit honestly for the odd-length canonical input.
The replay distinguishes swap frames from pointer-advance frames so the viewer can see $left and $right converge.

two pointers `$left` starts at index `0`, `$right` starts at `scalar(@arr) - 1`. Each loop iteration swaps `$arr[$left]` and `$arr[$right]` and moves the pointers toward each other.