Rearrange the elements of one array according to an index in another array without using additional space in memory. Note: This solution works but destroys the positions array in the process!

rearrange.rb

characters = [ 'o', 'h', 'e', 'd', 'n', 'r', 'y', 'g', 'a', 'b', 'e' ]
positions = [ 1, 0, 4, 2, 6, 9, 8, 3, 7, 10, 5]

def rearrange(characters, positions)
  positions.length.times do |i|
    positions[i] = characters[positions[i]]
  end
  positions.each_with_index {|e, i| characters[i] = e}
end

rearrange(characters, positions)

Alas, the space requirement makes this more functional solution a no-go:

def rearrange(characters, positions)
  positions.zip(characters).sort.map(&:last)
end

Positions array be damned!

def rearrange(characters, positions)
  positions.each_with_index do |obj, index|
    positions[index] = characters[obj]
  end

  positions
end