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.rb

arr = [1, 2, 3, 4, 5, 6, 7]
left = 0
right = arr.length - 1
while left < right
	tmp = arr[left]
	arr[left] = arr[right]
	arr[right] = tmp
	left = left + 1
	right = right - 1
end
puts arr.inspect

Complexity

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

Implementation notes

Ruby: explicit three-line tmp = arr[left]; arr[left] = arr[right]; arr[right] = tmp swap keeps the move visible. The stdlib arr.reverse (or the in-place arr.reverse!) would hide the lesson entirely, and arr[left], arr[right] = arr[right], arr[left] parallel assignment would collapse the swap into a single frame.
left = 0 and right = arr.length - 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 shows both left and right, the values about to be swapped, and the array contents after the swap. The loop-exit frame is the moment the pointers meet.

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