Cahnges overview:

Add HeapSort.

CodingInterview.Tests/CodingInterview.Tests.csproj

@@ -36,6 +36,7 @@
     <Compile Include="Sort\SelectionSortTests.cs" />
     <Compile Include="Sort\MergeSortTests.cs" />
     <Compile Include="Sort\QuickSortTests.cs" />
+    <Compile Include="Sort\HeapSortTests.cs" />
   </ItemGroup>
   <ItemGroup>
     <None Include="packages.config" />

CodingInterview.Tests/Sort/HeapSortTests.cs

+// 
+//
+// This file - HeapSortTests.cs was created by Dmytro Bogatov (dmytro@dbogatov.org)
+// on 12/7/2016, 2:47 PM
+using System;
+using System.Linq;
+using NUnit.Framework;
+
+namespace Sort
+{
+	[TestFixture]
+	public class HeapSortTests
+	{
+		[Test]
+		public void TestCases()
+		{
+			Assert.AreEqual(
+				new int[] { -1, 1, 2, 5, 8, 9, 45 },
+				HeapSort.Sort(new int[] { 5, 2, 1, -1, 8, 45, 9 })
+			);
+
+			Assert.AreEqual(
+				new int[] { 2, 5, 6, 8, 9 },
+				HeapSort.Sort(new int[] { 5, 2, 6, 8, 9 })
+			);
+
+			Assert.AreEqual(
+				new int[] { 1 },
+				HeapSort.Sort(new int[] { 1 })
+			);
+
+			Assert.AreEqual(
+				new int[] { },
+				HeapSort.Sort(new int[] { })
+			);
+
+			Assert.AreEqual(
+				new int[] { 5, 5, 6 },
+				HeapSort.Sort(new int[] { 5, 6, 5 })
+			);
+
+
+			Random randNum = new Random();
+			var list = Enumerable.Repeat(0, 25).Select(i => randNum.Next(-100, 100)).ToList();
+			var input = list.ToArray();
+			list.Sort();
+			var expected = list.ToArray();
+
+			Assert.AreEqual(
+				expected,
+				HeapSort.Sort(input)
+			);
+		}
+	}
+}
+
+

CodingInterview/CodingInterview.csproj

@@ -36,6 +36,7 @@
     <Compile Include="Sort\SelectionSort.cs" />
     <Compile Include="Sort\MergeSort.cs" />
     <Compile Include="Sort\QuickSort.cs" />
+    <Compile Include="Sort\HeapSort.cs" />
   </ItemGroup>
   <ItemGroup>
     <Folder Include="Sort\" />

CodingInterview/Sort/HeapSort.cs

+// 
+//
+// This file - HeapSort.cs was created by Dmytro Bogatov (dmytro@dbogatov.org)
+// on 12/7/2016, 2:44 PM
+
+namespace Sort
+{
+	// Mostly borrowed from https://begeeben.wordpress.com/2012/08/21/heap-sort-in-c/
+	public class HeapSort
+	{
+		public static int[] Sort(int[] input)
+		{
+			if (input.Length < 2)
+			{
+				return input;
+			}
+
+			//Build-Max-Heap
+			int heapSize = input.Length;
+			for (int p = (heapSize - 1) / 2; p >= 0; p--)
+				MaxHeapify(input, heapSize, p);
+
+			for (int i = input.Length - 1; i > 0; i--)
+			{
+				//Swap
+				int temp = input[i];
+				input[i] = input[0];
+				input[0] = temp;
+
+				heapSize--;
+				MaxHeapify(input, heapSize, 0);
+			}
+
+			return input;
+		}
+
+		static void MaxHeapify(int[] input, int heapSize, int index)
+		{
+			int left = (index + 1) * 2 - 1;
+			int right = (index + 1) * 2;
+			int largest = 0;
+
+			if (left < heapSize && input[left] > input[index])
+				largest = left;
+			else
+				largest = index;
+
+			if (right < heapSize && input[right] > input[largest])
+				largest = right;
+
+			if (largest != index)
+			{
+				int temp = input[index];
+				input[index] = input[largest];
+				input[largest] = temp;
+
+				MaxHeapify(input, heapSize, largest);
+			}
+		}
+
+	}
+}
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