Add day 1 solution.

day1.cr

@@ -0,0 +1,27 @@
+INPUT = File.read("day1.txt").lines.map(&.chomp.to_i32) 
+
+def part1
+  input = INPUT.clone
+  input.each_with_index do |v, i|
+    input.each_with_index do |v2, j|
+      next unless v + v2 == 2020 && i != j
+      puts v*v2
+    end
+  end
+end
+
+def part2
+  input = INPUT.clone
+  input.each_with_index do |v, i|
+    input.each_with_index do |v2, j|
+      next if j == i
+      input.each_with_index do |v3, k|
+        next if k == j || k == i
+        puts v3*v2*v if v3 + v2 + v == 2020
+      end
+    end
+  end
+end
+
+part1
+part2