chore(CSharp): added single number determination (#488)

algorithms/CSharp/README.md

@@ -1,6 +1,9 @@
 # C#
 For running the `.cs` file please using [.Net Finddle](https://dotnetfiddle.net/)
 
+## Arrays
+1. [Single Number](src/Arrays/single-number.cs)
+
 ## Sorts
 
 1. [Bubble Sort](src/Sorts/bubble-sort.cs)

algorithms/CSharp/src/Arrays/single-number.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Collections.Generic;
+
+namespace Algorithms.Arrays
+{
+    public class SingleNumber
+    {
+        public static int DetermineSingleNumber(List<int> numbers)
+        {
+            int single = numbers[0];
+            for(int i = 1; i < numbers.Count; i++)
+            {
+                single ^= numbers[i];
+            }
+
+            return single;
+        }
+
+        public static void Main()
+        {
+            List<int> numbers = new List<int> { 7, 1, 2, 3, 1, 7, 2 };
+            Console.WriteLine(DetermineSingleNumber(numbers));
+        }
+    }
+}
+
+
+/*
+ * Given a non-empty list of integers, every element except one appears twice
+ * Find out the single one
+ * Time complexity: O(n)
+ * Space complexity: O(1)
+ */

algorithms/CSharp/test/Arrays/single-number.cs

@@ -0,0 +1,35 @@
+using NUnit.Framework;
+using System.Collections.Generic;
+
+namespace Algorithms.Tests.Arrays
+{
+    [TestFixture]
+    class SingleNumber
+    {
+        static object[] TestCasesForSingleNumber =
+        {
+            new object[]
+            {
+                new List<int> { 7, 2, 2, 1, 3, 7, 3},
+                1
+            },
+            new object[]
+            {
+                new List<int> { 118 },
+                118
+            },
+            new object[]
+            {
+                new List<int> { 11, 12, 13, 14, 13, 12, 11},
+                14
+            }
+        };
+
+        [TestCaseSource(nameof(TestCasesForSingleNumber))]
+        public void TestSingleNumber_ShouldGetExpectedResult(List<int> numbers, int expected)
+        {
+            int result = Algorithms.Arrays.SingleNumber.DetermineSingleNumber(numbers);
+            Assert.AreEqual(expected, result);
+        }
+    }
+}