Day 4

Day4/Day4.cs

@@ -0,0 +1,63 @@
+using System;
+using System.Linq;
+
+namespace Advent
+{
+    public static class Day4
+    {
+        public static void Execute()
+        {
+            //Console.WriteLine(IsValidPassword("112233"));
+            //Console.WriteLine(IsValidPassword("123444"));
+            //Console.WriteLine(IsValidPassword("111122"));
+
+            var count = 0;
+
+            for (var i = 109165; i <= 576723; i++)
+            {
+                if (IsValidPassword(i.ToString()))
+                    count++;
+            }
+
+            Console.WriteLine(count);
+        }
+
+        private static bool IsValidPassword(string password)
+        {
+            var digits = password.ToCharArray();
+
+            var sortedDigits = password.ToCharArray().OrderBy(c => c);
+
+            if (!digits.SequenceEqual(sortedDigits))
+                return false;
+
+            var hasDouble = false;
+            var index = 0;
+            var letter = digits[0];
+
+            while (index < 6)
+            {
+                var count = 1;
+
+                while (index < 5 && digits[index + 1] == letter)
+                {
+                    count++;
+                    index++;
+                }
+
+                if (count == 2)
+                {
+                    hasDouble = true;
+                    break;
+                }
+
+                index++;
+
+                if (index < 6)
+                    letter = digits[index];
+            }
+
+            return hasDouble;
+        }
+    }
+}