pass ball in ruby

compute.rb

# ruby

def compute(current,remain)
  result = 0
  if remain == 0
    result = current == 'a' ? 1 : 0
  else
    for n in ['a','b','c','d']      
      result += compute(n,remain-1) if n != current
    end
  end
  return result
end

puts compute('a',5)