c#: List.Sort()实现稳固排序(stable sort)

时间：2017-11-04 19:32:49 阅读：308 评论：0 收藏：0 [点我收藏+]

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1. 源起：

KV 7.0加入列表管理功能，处理排序问题时，对空列表执行按大小、日期、长度排序时，其中次序会发生改变，令人纳闷。

没天理呀，不应该啊！List.Sort()方法，它为什么？

对此问题，深入去了解，倒发现了有趣的问题：稳固排序与非稳固排序。

2、稳固排序与非稳固排序

Remarks

If comparison is provided, the elements of the List<T> are sorted using the method represented by the delegate.

If comparison is null, an ArgumentNullException is thrown.

This method uses Array.Sort, which applies the introspective sort as follows:

If the partition size is fewer than 16 elements, it uses an insertion sort algorithm

If the number of partitions exceeds 2 * LogN, where N is the range of the input array, it uses a Heapsort algorithm.

Otherwise, it uses a Quicksort algorithm.

This implementation performs an unstable sort; that is, if two elements are equal, their order might not be preserved. In contrast, a stable sort preserves the order of elements that are equal.

On average, this method is an O(n log n) operation, where n is Count; in the worst case it is an O(n ^ 2) operation.

大意是此实现将执行不稳定排序。也就是说，如果两个元素相等，则可能不会保留其顺序。

我们建Demo做验证，用例如下：

    var list = new List<string>();
    list.AddRange(new string[] { "3", "2", "1", "4" });

    list.Sort((x, y) =>
    {
        return 0;
    });

    foreach (string s in list)
        Console.WriteLine(s);

其输出为：

技术分享

验证上述结果，推测它是使用了二分法快速反序，把后半部分置前了。

可八辈的我们宁愿它输入为原始顺序3214，这个合理，但是，可是，它无耻的变了……

3、实际场景

比如，我们需要对特定的对象进行排序，如下摘取网上例子稍加修正，以做示例：

    static void Main(string[] args)
    {
        var p1 = new Person() { Name = "Abby", Age = 38 };
        var p4 = new Person() { Name = "Jason", Age = 23 };
        var p2 = new Person() { Name = "Bob", Age = 23 };
        var p3 = new Person() { Name = "Charlie", Age = 23 };
        var p5 = new Person() { Name = "Danielle", Age = 18 };

        var list = new List<Person>();
        list.Add(p1);
        list.Add(p2);
        list.Add(p3);
        list.Add(p4);
        list.Add(p5);
        list.Sort();

        Console.WriteLine("Unstable List Sort:");
        foreach (Person p in list)
            Console.WriteLine(p);

        Console.ReadLine();
    }
    
    class Person : IComparable
    {
        public string Name { get; set; }
        public int Age { get; set; }
        
        public int CompareTo(object obj)
        {
            int result = 1;
            if (obj != null && obj is Person)
            {
                var person = (Person)obj;
                result = this.Age.CompareTo(person.Age);;
            }
            return result;
        }
        
        public override string ToString()
        {
            return String.Format("{0} - {1}", this.Name, this.Age);
        }
    }

其输出为：

技术分享

其中间23岁之三项，没有按Jason、Bob、Charlie的次序来，甚至也不是倒序。

我就是想要它加入顺序不变，怎么办？

对此问题，搜出一些文章，有建议用LINQ排序因为它是稳固排序，可我们工程所用.net framework 2.0，不支持，只好做罢。

在stackoverflow上，碰到一群难兄难弟，诸般建议阅过，找到可用方法，加索引！

4、解决方法

且修改Person类，加入SortIndex属性如下，并修正其比较函数：

    class Person : IComparable
    {
        public string Name { get; set; }
        public int Age { get; set; }
        public int SortIndex { get; set; }

        public int CompareTo(object obj)
        {
            int result = 1;
            if (obj != null && obj is Person)
            {
                var person = (Person)obj;
                result = this.Age.CompareTo(person.Age);
                
                if (result == 0)
                    result = this.SortIndex.CompareTo(person.SortIndex);
            }
            return result;
        }
        ...
    }

初始化时，加入其索引：

        var p1 = new Person() { Name = "Abby", Age = 38, SortIndex = 0 };
        var p4 = new Person() { Name = "Jason", Age = 23, SortIndex = 1 };
        var p2 = new Person() { Name = "Bob", Age = 23, SortIndex = 2 };
        var p3 = new Person() { Name = "Charlie", Age = 23, SortIndex = 3 };
        var p5 = new Person() { Name = "Danielle", Age = 18, SortIndex = 4 };