Optimize day 5 solution.

day5.cr

@@ -2,30 +2,25 @@ require "./common.cr"
 
 lines = File.read("day5_input").split("\n")
 
+struct Char
+  def inverse?(other)
+    return (other != self) && (other.downcase == self.downcase)
+  end
+end
+
 def react(string)
-  string = string.chars
+  stack = [] of Char
 
-  changed = true
-  while changed
-    changed = false
-    i = 0
-    while i < string.size - 1
-      next_i = i + 1
-      if string[i] != string[next_i] &&
-          string[i].downcase == string[next_i].downcase
-        string.delete_at i
-        string.delete_at i
-
-        i -= 1
-        i = 0 if i < 0
-        changed = true
+  string.each_char do |char|
+    last_char = stack.last?
+    if !last_char || !last_char.inverse? char
+      stack << char
     else
-        i += 1
-      end
+      stack.pop
     end
   end
 
-  return string
+  return stack
 end
 
 puts react(lines[0]).size