维护排序顺序C＃的集合

Question

I have a class Foo which contains a list of objects: List<Bar> . Each Bar has a property which they can be ordered on (of type TimeSpan , representing a duration), and Bar is an immutable object - that is, the duration does not change over the running of the algorithm. At the moment, for each Foo I also maintain the Bar that would be first in the list if it were to be ordered (ie the Bar of shortest duration). Something like this:

public class Foo
{
    public List<Bar> AllBars { get; set; }

    public Bar FirstBar { get; set; }

    public Foo (Bar bar)
    {
        FirstBar = bar;

        AllBars = new List<Bar>() { bar };
    }

    public AddBar(Bar bar)
    {
        if(bar.Duration < FirstBar.Duration)
        {
            FirstBar = bar;
        }

        AllBars.Add(bar);
    }
}

This class Foo is used in an algorithm where processing performance (speed) is critical. Memory is important but not as much as speed. There is a list of n Foo s, each of which has up to m Bar s. This class has served me well up until this point. I now wish to offer the user several choices, meaning I will need to provide random access to the first few Bar s in the list.

I would thus like to store my Bar s in order so that I can access them by index in order. In my Bar class I implemented IComparable to allow Bar s to be compared on duration but I am stuck at choosing an appropriate data type. I looked at System.Collections.SortedList but (unless I am wrong) this appears to reference elements by key as it implements IDictionary . What collection could I use that would maintain my objects such that they stay sorted, and such that they are traversable in order of index?

Answer 1

I prefer to use SortedSet<T> , which is a binary tree where the key and value are the same object. This once again means that adding/removing/lookups are logarithmic - O(log n) - but you gain the ability to iterate over the items in order. For this collection to be effective, type T must implement IComparable<T> or you need to supply an external IComparer<T> .

Answer 2

Why not just use List.Insert ?
Insert is O(n) and lets you insert at a specific index.
n + n is still O(n)

public AddBar(Bar bar)
{
    int index = 0;    
    foreach (bar b in AllBar)
    {
       if(bar.Duration < b.Duration)
         break;
       index++;
    }
    AllBars.Insert(index, bar);
}

So you have sort and O(1) index
At a cost of O(n) Add
The current Add in also O(n)

A SortedList in NlogN and then you don't have index as the key is Duration and the key in not unique

A SortedSet insert is LogN but a ToList is O(n) so you are still O(n)

Calling the the Sort method on the list is NlogN

This answers the stated question of: What collection could I use that would maintain my objects such that they stay sorted, and such that they are traversable in order of index?

I don't think you are going to do that with better than O(n) Add.
Who ever gave it a down vote then what is a better solution?

Answer 3

(promoted from a comment, as requested by the asker)

If you can live with having "values" that mean nothing, just use a SortedList<Bar, object> where you do not use the value part.

Add with yourSortedList.Add(yourBar, null) in O(n) time (the list will have to move "up" all entries after the point where you insert). Retrieve the i th entry in O(1) time with yourSortedList.Keys[i] .

See the SortedList<,>.Keys property documentation for some "proof" that the above description is correct. Note that a SortedList<,> actually consists of a "list" (ie an array of length Capacity , subject to substitution by a larger array when necessary). This is different from SortedDictionary<,> which I believe is a binary search tree.

Note however: You will not be able to have duplicates in your SortedList<,> , so two members in the list are not allowed to CompareTo each other with return value zero.