ruby quicksort

quicksort.rb

def quicksort(a, first, last)
  if first < last
    p_index = partition(a, first, last)
    quicksort(a, first, p_index-1)
    quicksort(a, p_index+1, last)
  end
  return a
end

def partition(a, first, last)
  pivot = a[last]
  p_index = first
  for i in first...last do
    if a[i] < pivot
      a[i], a[p_index] = a[p_index], a[i]
      p_index += 1
    end
  end
  a[p_index], a[last] = a[last], a[p_index]
  return p_index
end

a = [3,4,6,7,8,9,5,6,1,7]
puts quicksort(a, 0, a.length-1)