gistfile1.rb

def max_length_histogram(str)
  Hash.new(0).tap do |hist|
    str.scan(/((.)\2*)/) do |substring, letter|
      hist[letter] = [ hist[letter], substring.size ].max
    end
  end
end

def longest_run(str)
  max_length_histogram(str).max_by {|letter, n| n }
end

str = "aabbbbcccdebbbbbff"
p longest_run(str)        # ["b", 5]