Add day 5 solution.

day5.cr

@@ -0,0 +1,42 @@
+require "./common.cr"
+
+lines = File.read("day5_input").split("\n")
+
+def react(string)
+  lower = string.downcase.chars
+  string = string.chars
+
+  changed = true
+  while changed
+    changed = false
+    i = 0
+    while i < string.size - 1
+      next_i = i + 1
+      if string[i] != string[next_i] &&
+          lower[i] == lower[next_i]
+        string.delete_at i
+        string.delete_at i
+        lower.delete_at i
+        lower.delete_at i
+
+        i -= 1
+        i = 0 if i < 0
+        changed = true
+      else
+        i += 1
+      end
+    end
+  end
+
+  return string
+end
+
+puts react(lines[0]).size
+
+pairs = {} of Tuple(Char, Char) => Int32
+('a'..'z').to_a.zip(('A'..'Z').to_a).each do |pair|
+  new_string = lines[0].delete(pair[0]).delete(pair[1])
+  pairs[pair] = react(new_string).size
+end
+
+puts pairs.values.min