Solution for Day 15

day_15/both.rb

@@ -0,0 +1,22 @@
+def run(start, target)
+  turn = start.size
+  number = start.last
+  numbers = Array.new(target) { 0 }
+
+  start[0..-2].each_with_index do |value, turn|
+    numbers[value] = turn + 1
+  end
+
+  (turn..target - 1).each do |turn|
+    next_num = numbers[number].eql?(0) ? 0 : turn - numbers[number]
+
+    numbers[number] = turn
+    number = next_num
+  end
+  number
+end
+
+input = [19, 0, 5, 1, 10, 13]
+
+puts run input, 2020
+puts run input, 30_000_000