Question

.NET的Array.Sort()方法使用的排序算法是stable算法吗？

Answer 1

来自MSDN：

此实现执行不稳定的排序;也就是说，如果两个元素相等，则可能不会保留它们的顺序。相反，稳定的排序保留了相等元素的顺序。

排序使用内省排序。（Quicksort在4.0及更早版本的.NET框架中）。

如果您需要稳定排序，可以使用Enumerable.OrderBy。

Answer 2

通过Enumerable.OrderBy和Enumerable.ThenBy在LINQ中进行排序是一种稳定的排序实现，可以用作Array.Sort的替代方法。来自Enumerable.OrderBy documentation over at MSDN：

此方法执行稳定排序; 也就是说，如果两个元素的键是相等的，元素的顺序保留下来。相比之下，不稳定 sort不保留顺序具有相同密钥的元素。

此外，任何不稳定的排序实现（如Array.Sort的实现）都可以通过使用源序列或数组中元素的位置作为附加键来稳定，以作为平局。下面是一个这样的实现，作为任何一维数组上的通用扩展方法，并将Array.Sort转换为稳定的排序：

using System;
using System.Collections.Generic;

public static class ArrayExtensions {

    public static void StableSort<T>(this T[] values, Comparison<T> comparison) {
        var keys = new KeyValuePair<int, T>[values.Length];
        for (var i = 0; i < values.Length; i++)
            keys[i] = new KeyValuePair<int, T>(i, values[i]);
        Array.Sort(keys, values, new StabilizingComparer<T>(comparison));
    }

    private sealed class StabilizingComparer<T> : IComparer<KeyValuePair<int, T>> 
    {
        private readonly Comparison<T> _comparison;

        public StabilizingComparer(Comparison<T> comparison) {
            _comparison = comparison;
        }

        public int Compare(KeyValuePair<int, T> x, 
                           KeyValuePair<int, T> y) {
            var result = _comparison(x.Value, y.Value);
            return result != 0 ? result : x.Key.CompareTo(y.Key);
        }
    }
}

以下是使用上述StableSort的示例程序：

static class Program 
{
    static void Main() 
    {
        var unsorted = new[] {
            new Person { BirthYear = 1948, Name = "Cat Stevens" },
            new Person { BirthYear = 1955, Name = "Kevin Costner" },
            new Person { BirthYear = 1952, Name = "Vladimir Putin" },
            new Person { BirthYear = 1955, Name = "Bill Gates" },
            new Person { BirthYear = 1948, Name = "Kathy Bates" },
            new Person { BirthYear = 1956, Name = "David Copperfield" },
            new Person { BirthYear = 1948, Name = "Jean Reno" },
        };

        Array.ForEach(unsorted, Console.WriteLine);

        Console.WriteLine();
        var unstable = (Person[]) unsorted.Clone();
        Array.Sort(unstable, (x, y) => x.BirthYear.CompareTo(y.BirthYear));
        Array.ForEach(unstable, Console.WriteLine);

        Console.WriteLine();
        var stable = (Person[]) unsorted.Clone();
        stable.StableSort((x, y) => x.BirthYear.CompareTo(y.BirthYear));
        Array.ForEach(stable, Console.WriteLine);
    }
}

sealed class Person 
{
    public int BirthYear { get; set; }
    public string Name { get; set; }

    public override string ToString() {
        return string.Format(
            "{{ BirthYear = {0}, Name = {1} }}", 
            BirthYear, Name);
    }
}

以下是上述示例程序的输出（在安装了Windows Vista SP1和.NET Framework 3.5 SP1的计算机上运行）：

{ BirthYear = 1948, Name = Cat Stevens }
{ BirthYear = 1955, Name = Kevin Costner }
{ BirthYear = 1952, Name = Vladimir Putin }
{ BirthYear = 1955, Name = Bill Gates }
{ BirthYear = 1948, Name = Kathy Bates }
{ BirthYear = 1956, Name = David Copperfield }
{ BirthYear = 1948, Name = Jean Reno }

{ BirthYear = 1948, Name = Jean Reno }
{ BirthYear = 1948, Name = Kathy Bates }
{ BirthYear = 1948, Name = Cat Stevens }
{ BirthYear = 1952, Name = Vladimir Putin }
{ BirthYear = 1955, Name = Bill Gates }
{ BirthYear = 1955, Name = Kevin Costner }
{ BirthYear = 1956, Name = David Copperfield }

{ BirthYear = 1948, Name = Cat Stevens }
{ BirthYear = 1948, Name = Kathy Bates }
{ BirthYear = 1948, Name = Jean Reno }
{ BirthYear = 1952, Name = Vladimir Putin }
{ BirthYear = 1955, Name = Kevin Costner }
{ BirthYear = 1955, Name = Bill Gates }
{ BirthYear = 1956, Name = David Copperfield }

Answer 3

正如其他答案所述，Array.Sort不稳定。但是，LINQ OrderBy方法（和OrderByDescending等）是稳定的，这可能非常有用。

Answer 4

不，isn't：

此方法使用QuickSort算法。此实现执行不稳定的排序

Answer 5

UPDATE：此代码不稳定Array.Sort（确保元素始终按相同顺序排序）：

public static class ComparisonExtensions
{
    public static Comparison<T> WithGetHashCode<T>(this Comparison<T> current)
    {
        return (x, y) =>
        {
            var result = current(x, y);
            if (result == 0)
                return x.GetHashCode() - y.GetHashCode();
            return result;
        };
    }
}

使用：

Comparison<Person> comparison = (x, y) => x.BirthYear.CompareTo(y.BirthYear);
Array.Sort(unstable, comparison.WithGetHashCode());

.NET的`Array.Sort（）`方法使用的排序算法是一种稳定的算法吗？

5 个答案: