Add solutions for day 2.

day2_1.cr

@@ -0,0 +1,15 @@
+lines = File.read("day2").split("\n")
+lines.pop
+
+CHARS = ('a'..'z').to_a
+
+def has_n?(string, n)
+  CHARS.each do |c|
+    return true if string.count(c) == n
+  end
+  return false
+end
+
+two = lines.count { |s| has_n?(s, 2) }
+three = lines.count { |s| has_n?(s, 3) }
+puts two * three

day2_2.cr

@@ -0,0 +1,15 @@
+lines = File.read("day2").split("\n")
+lines.pop
+
+def count_changes(s1, s2)
+  pairs = s1.chars.zip s2.chars
+  total = 0
+  pairs.each do |pair|
+    total += 1 if pair[0] != pair[1]
+  end
+  return total
+end
+
+lines.product(lines).each do |pair|
+  puts "#{pair[0]}, #{pair[1]}" if count_changes(pair[0], pair[1]) == 1
+end