Add Lonely Integer.

src/hacker-rank/LonelyInteger.cs

+namespace CodingInterview.HackerRank
+{
+	/// <summary>
+	/// You will be given an array of integers.
+	/// All of the integers except one occur twice.
+	/// That one is unique in the array.
+	///
+	/// Given an array of integers, find and print the unique element.
+	///
+	/// For example, a = [1, 2, 3, 4, 3, 2, 1], the unique element is 4.
+	/// </summary>
+	public class LonelyInteger
+	{
+		/// <summary>
+		/// Finds the unique element
+		/// </summary>
+		/// <param name="a">An array of integers where each integer occurs twice except one, which occurs once</param>
+		/// <returns>The integer which occurs once</returns>
+		public long Solve(int[] a)
+		{
+			for (var i = 1; i < a.Length; i++)
+			{
+				a[0] = a[0] ^ a[i];
+			}
+			return a[0];
+		}
+	}
+}

test/hacker-rank/LonelyInteger.cs

+using Xunit;
+
+namespace CodingInterview.Tests.HackerRank
+{
+	public class LonelyInteger
+	{
+		[Fact]
+		public void TestCases()
+		{
+			// From Hacker Rank
+			Assert.Equal(4, new CodingInterview.HackerRank.LonelyInteger().Solve(new int[] { 1, 2, 3, 4, 3, 2, 1 }));
+		}
+	}
+}