From aef1ed2808c3e75658f0cf341102dc43a8998d0d Mon Sep 17 00:00:00 2001
From: Danila Fedorin <danila.fedorin@gmail.com>
Date: Mon, 30 Nov 2020 21:58:36 -0800
Subject: [PATCH] Add two-pointer solution for day 1.

---
 day1.cr | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/day1.cr b/day1.cr
index 78015cb..6373b06 100644
--- a/day1.cr
+++ b/day1.cr
@@ -1,11 +1,18 @@
 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
+  input = INPUT.clone.sort!
+  bottom = 0
+  top = input.size - 1
+  loop do
+    sum = input[bottom] + input[top]
+    if sum == 2020
+      puts input[bottom] * input[top] 
+      return
+    elsif sum > 2020
+      top -= 1
+    else
+      bottom += 1
     end
   end
 end