Day01_Part2: Add failing tests.

AdventOfCode2018/Day01.cs

@@ -1,4 +1,6 @@
-namespace AdventOfCode2018
+using System;
+
+namespace AdventOfCode2018
 {
     /*
      --- Day 1: Chronal Calibration ---
@@ -29,6 +31,32 @@
         -1, -2, -3 results in -6
 
     Starting with a frequency of zero, what is the resulting frequency after all of the changes in frequency have been applied?
+
+    --- Part Two ---
+
+    You notice that the device repeats the same frequency change list over and over. To calibrate the device, you need to find the first frequency it reaches twice.
+
+    For example, using the same list of changes above, the device would loop as follows:
+
+        Current frequency  0, change of +1; resulting frequency  1.
+        Current frequency  1, change of -2; resulting frequency -1.
+        Current frequency -1, change of +3; resulting frequency  2.
+        Current frequency  2, change of +1; resulting frequency  3.
+        (At this point, the device continues from the start of the list.)
+        Current frequency  3, change of +1; resulting frequency  4.
+        Current frequency  4, change of -2; resulting frequency  2, which has already been seen.
+
+    In this example, the first frequency reached twice is 2. Note that your device might need to repeat its list of frequency changes many times before a duplicate frequency is found, and that duplicates might be found while in the middle of processing the list.
+
+    Here are other examples:
+
+        +1, -1 first reaches 0 twice.
+        +3, +3, +4, -2, -4 first reaches 10 twice.
+        -6, +3, +8, +5, -6 first reaches 5 twice.
+        +7, +7, -2, -7, -4 first reaches 14 twice.
+
+    What is the first frequency your device reaches twice?
+
     */
 
     public class Day01
@@ -54,5 +82,10 @@
 
             return accumulator.ToString();
         }
+
+        public string ResolveDay01_Part2(string[] inputs)
+        {
+            throw new NotImplementedException();
+        }
     }
 }