Add day15 solution.

day15.cr

@@ -0,0 +1,37 @@
+require "advent"
+INPUT = input(2020, 15).split(",").map(&.to_i32) 
+
+def run(input, times)
+  ls = {} of Int32 => {Int32,Int32}
+  last = 0
+  input.each_with_index do |n, i|
+    ls[n] ||= {-1,-1}
+    f,s = ls[n]
+    ls[n] = {i,f}
+    last = n
+  end
+  count = input.size
+  while count < times
+    n = 0
+    if ls[last][1] != -1
+      n = ls[last][0] - ls[last][1]
+    end
+    last = n
+    ls[n] ||= {-1,-1}
+    f,s = ls[n]
+    ls[n] = {count,f}
+    count += 1
+  end
+  last
+end
+
+def part1(input)
+  run(input, 2020)
+end
+
+def part2(input)
+  run(input, 30000000)
+end
+
+puts part1(INPUT.clone)
+puts part2(INPUT.clone)