算法：堆排序（Heap Sort）

备注

考虑到 Heap 的特性，很容易想到将其用作排序的用处，为了提高效率需要适当的改进一下，如：in place remove 和 in place move down。

代码

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;

 namespace DataStuctureStudy.Sorts
 {
     class HeapSort<T>
         where T : IComparable<T>
     {
         public static void Test()
         {
             var items = new int[] { , , , , , , , , ,  };
             HeapSort<int>.Sort(items);
             Console.WriteLine(String.Join(",", items));
         }

         public static void Sort(T[] items)
         {
             for (int i = items.Length /  - ; i >= ; i--)
             {
                 MoveDown(items, i, items.Length - );
             }

             for (var i = items.Length - ; i >= ; i--)
             {
                 items[i] = Remove(items, i);
             }
         }

         private static T Remove(T[] items, int endIndex)
         {
             var result = items[];
             items[] = items[endIndex];

             MoveDown(items, , endIndex - );

             return result;
         }

         private static void MoveDown(T[] items, int startIndex, int endIndex)
         {
             var top = items[startIndex];
             var current = startIndex;

             while (current <= endIndex)
             {
                 var large = ;
                 var left =  * current + ;
                 var right = left + ;

                 if (left <= endIndex && right <= endIndex)
                 {
                     if (items[left].CompareTo(items[right]) >= )
                     {
                         large = left;
                     }
                     else
                     {
                         large = right;
                     }
                 }
                 else if (left <= endIndex)
                 {
                     large = left;
                 }
                 else
                 {
                     break;
                 }

                 if (items[large].CompareTo(top) <= )
                 {
                     break;
                 }

                 items[current] = items[large];
                 current = large;
             }

             items[current] = top;
         }
     }
 }

备注

代码的一部分将未排序数组变为堆，然后将对堆执行 in place remove。