C#中Array.Sort() 快速排序-源码分析

快速排序是C.R.A.Hoare于1962年提出的一种划分交换排序。它采用了一种分治的策略，通常称其为分治法(Divide-and-ConquerMethod)。

该方法的基本思想是：

1．先从数列中取出一个数作为基准数。

2．分区过程，将比这个数大的数全放到它的右边，小于或等于它的数全放到它的左边。

3．再对左右区间重复第二步，直到各区间只有一个数。

之前提到过，Array的Sort()方法采用的是快速排序方法，通过使用.NET Reflector对程序集mscorlib进行解析，分析源码。

首先安装.NET Reflector，打开VS2010，新建一个项目，键入如下代码：

string [] names = {"wang", "li", "liu", "zhao"};

Array.Sort(names);

在Sort上右键，选择“Open in .NET Reflector Desktop”,便会自动打开.NET Reflector：

public static void Sort(T[] array)

{ if (array == null)

{

throw new ArgumentNullException("array");

}

Sort(array, array.GetLowerBound(0), array.Length, null);

}

点击Sort(array, array.GetLowerBound(0), array.Length, null)：

public static void Sort(T[] array, int index, int length, IComparer comparer)

{ if (array == null)

{ throw new ArgumentNullException("array");

}

if ((index < 0) || (length < 0))

{ throw new ArgumentOutOfRangeException((length < 0) ? "length" : "index", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));

}

if ((array.Length - index) < length)

{

throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));

}

if ((length > 1) && (((comparer != null) && (comparer != Comparer.Default)) || !TrySZSort(array, null, index, (index + length) - 1)))

{

ArraySortHelper.Default.Sort(array, index, length, comparer);

}

}

点击 ArraySortHelper.Default.Sort(array, index, length, comparer)：

internal class ArraySortHelper : IArraySortHelper

{

// Fields

private static IArraySortHelper defaultArraySortHelper;

// Methods

public ArraySortHelper();

public int BinarySearch(T[] array, int index, int length, T value, IComparer comparer);

[SecuritySafeCritical]

private static IArraySortHelper CreateArraySortHelper();

internal static int InternalBinarySearch(T[] array, int index, int length, T value, IComparer comparer);

internal static void QuickSort(T[] keys, int left, int right, IComparer comparer);

public void Sort(T[] keys, int index, int length, IComparer comparer);

private static void SwapIfGreaterWithItems(T[] keys, IComparer comparer, int a, int b);

// Properties

public static IArraySortHelper Default { get; }

}

此时，可以看到快速排序，点击QuickSort，得到快速排序的源码：

internal static void QuickSort(T[] keys, int left, int right, IComparer comparer)

{

do

{

int a = left;

int b = right;

int num3 = a + ((b - a) >> 1);

ArraySortHelper.SwapIfGreaterWithItems(keys, comparer, a, num3);

ArraySortHelper.SwapIfGreaterWithItems(keys, comparer, a, b);

ArraySortHelper.SwapIfGreaterWithItems(keys, comparer, num3, b);

T y = keys[num3];

do

{

while (comparer.Compare(keys[a], y) < 0)

{

a++;

}

while (comparer.Compare(y, keys[b]) < 0)

{

b--;

}

if (a > b)

{

break;

}

if (a < b)

{

T local2 = keys[a];

keys[a] = keys[b];

keys[b] = local2;

}

a++;

b--;

}

while (a <= b);

if ((b - left) <= (right - a))

{

if (left < b)

{

ArraySortHelper.QuickSort(keys, left, b, comparer);

}

left = a;

}

else

{

if (a < right)

{

ArraySortHelper.QuickSort(keys, a, right, comparer);

}

right = b;

}

}

while (left < right);

}

internal static void QuickSort(T[] keys, int left, int right, IComparer comparer)

{

do

{

int a = left;

int b = right;

int num3 = a + ((b - a) >> 1);

ArraySortHelper.SwapIfGreaterWithItems(keys, comparer, a, num3);

ArraySortHelper.SwapIfGreaterWithItems(keys, comparer, a, b);

ArraySortHelper.SwapIfGreaterWithItems(keys, comparer, num3, b);

T y = keys[num3];

do

{

while (comparer.Compare(keys[a], y) < 0)

{

a++;

}

while (comparer.Compare(y, keys[b]) < 0)

{

b--;

}

if (a > b)

{

break;

}

if (a < b)

{

T local2 = keys[a];

keys[a] = keys[b];

keys[b] = local2;

}

a++;

b--;

}

while (a <= b);

if ((b - left) <= (right - a))

{

if (left < b)

{

ArraySortHelper.QuickSort(keys, left, b, comparer);

}

left = a;

}

else

{

if (a < right)

{

ArraySortHelper.QuickSort(keys, a, right, comparer);

}

right = b;

}

}

while (left < right);

}