Let see two examples before we go further:

As above we can see, to reverse a array in-place, we just need swap each two elements in the array from both side to middle.

The pseudo code from WikiPedia:

function reverse_in_place(a[0..n-1])
	for i from 0 to floor((n-2)/2)
		tmp := a[i]
		a[i] := a[n − 1 − i]
		a[n − 1 − i] := tmp

The loop travels the array to the middle and swap each other in the list, two points we must be noticed:

  1. We stop the element at(not before): floor((n - 2) / 2), It’s both 1 of the two above figures.
  2. We swap current element with n - 1 - i.