Ruby QuickSort and QuickSelect

quick_select.rb

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

def quickselect(arr, first, last, i)
  return arr[first] if first == last
  p_index = partition(arr, first, last)
  k = p_index - first + 1
  if i == k
    arr[p_index]
  elsif i < k
    quickselect(arr, first, p_index - 1,  i)
  else  
    quickselect(arr, p_index + 1, last, i - k)
  end
end


def partition(arr, first, last)
  pivot = arr[last]
  p_index = first
  (first..last).each do |i|
    if arr[i] <= pivot
      arr[i], arr[p_index] = arr[p_index], arr[i]
      p_index += 1
    end
  end
  arr[last], arr[p_index] = arr[p_index], pivot
  p_index
end