Ruby: lowest common ancestor (recursive)

lca.rb

def lcs s1, s2  
  if s1.empty? or s2.empty?
    return ''
  elsif s1[0] == s2[0]
    return s1[0] + lcs(s1[1..-1], s2[1..-1])
  end
  left = lcs s1[1..-1], s2
  right = lcs s1, s2[1..-1]
  return left.size > right.size ? left : right
end