Add day 06: Frequent Characters

Author: petertseng

Diff for: 06_frequent_characters.rb

@@ -0,0 +1,4 @@
+by_freq = ARGF.map { |l| l.strip.chars }.transpose.map(&:tally).map { |f|
+  f.to_a.sort_by(&:last).map(&:first).freeze
+}.freeze
+puts %i(last first).map { |s| by_freq.map(&s).join }