def f(L): N = len(L) for k in range(1, N//2 + 1): i = k - 1 j = N - k while i < j: # swap elements: L[i], L[j] = L[j], L[i] # update i, j: i += 1 j -= 1 return L assert f([16, 14, 12, 7, 9, 11]) == [11, 14, 7, 12, 9, 16]
assert f(??) == [11, 14, 7, 12, 9, 16]
assert f([16, 14, 12, 7, 9, 11]) == ??