Ruby Merge Sort

Ruby Merge Sort

def mergesort( a )
     n = a.size
     return a if ( n == 1 )

     l1 = a[0...(n/2)]
     l2 = a[(n/2)..n]

     l1 = mergesort( l1 )
     l2 = mergesort( l2 )

     return merge( l1, l2 )
end

def merge( left, right )
  if left.empty?
     right
  elsif right.empty?
     left
  elsif left.first < right.first
     [left.first] + merge(left[1..left.length], right)
  else
     [right.first] + merge(left, right[1..right.length])
  end
end

puts mergesort([7,4,3,6,8,2,1,9,5]).join(' ')