最快的方法是使用 HashSet
。
对于 HashSet
的 Contains
是 O(1)。
我拿了你的代码,并为 HashSet<int>
添加了基准测试。
HashSet<int> set = new HashSet<int>(list);
的性能成本几乎为零。
代码
void Main()
{
ContainsExistsAnyVeryShort();
ContainsExistsAnyShort();
ContainsExistsAny();
}
private static void ContainsExistsAny()
{
Console.WriteLine("***************************************");
Console.WriteLine("********* ContainsExistsAny ***********");
Console.WriteLine("***************************************");
List<int> list = new List<int>(6000000);
Random random = new Random();
for (int i = 0; i < 6000000; i++)
{
list.Add(random.Next(6000000));
}
int[] arr = list.ToArray();
HashSet<int> set = new HashSet<int>(list);
find(list, arr, set, (method, stopwatch) => $"{method}: {stopwatch.ElapsedMilliseconds}ms");
}
private static void ContainsExistsAnyShort()
{
Console.WriteLine("***************************************");
Console.WriteLine("***** ContainsExistsAnyShortRange *****");
Console.WriteLine("***************************************");
List<int> list = new List<int>(2000);
Random random = new Random();
for (int i = 0; i < 2000; i++)
{
list.Add(random.Next(6000000));
}
int[] arr = list.ToArray();
HashSet<int> set = new HashSet<int>(list);
find(list, arr, set, (method, stopwatch) => $"{method}: {stopwatch.ElapsedMilliseconds}ms");
}
private static void ContainsExistsAnyVeryShort()
{
Console.WriteLine("*******************************************");
Console.WriteLine("***** ContainsExistsAnyVeryShortRange *****");
Console.WriteLine("*******************************************");
List<int> list = new List<int>(10);
Random random = new Random();
for (int i = 0; i < 10; i++)
{
list.Add(random.Next(6000000));
}
int[] arr = list.ToArray();
HashSet<int> set = new HashSet<int>(list);
find(list, arr, set, (method, stopwatch) => $"{method}: {stopwatch.ElapsedTicks} ticks");
}
private static void find(List<int> list, int[] arr, HashSet<int> set, Func<string, Stopwatch, string> format)
{
Random random = new Random();
int[] find = new int[10000];
for (int i = 0; i < 10000; i++)
{
find[i] = random.Next(6000000);
}
Stopwatch watch = Stopwatch.StartNew();
for (int rpt = 0; rpt < 10000; rpt++)
{
list.Contains(find[rpt]);
}
watch.Stop();
Console.WriteLine(format("List/Contains", watch));
watch = Stopwatch.StartNew();
for (int rpt = 0; rpt < 10000; rpt++)
{
list.Exists(a => a == find[rpt]);
}
watch.Stop();
Console.WriteLine(format("List/Exists", watch));
watch = Stopwatch.StartNew();
for (int rpt = 0; rpt < 10000; rpt++)
{
list.Any(a => a == find[rpt]);
}
watch.Stop();
Console.WriteLine(format("List/Any", watch));
watch = Stopwatch.StartNew();
for (int rpt = 0; rpt < 10000; rpt++)
{
arr.Contains(find[rpt]);
}
watch.Stop();
Console.WriteLine(format("Array/Contains", watch));
watch = Stopwatch.StartNew();
for (int rpt = 0; rpt < 10000; rpt++)
{
Array.Exists(arr, a => a == find[rpt]);
}
watch.Stop();
Console.WriteLine(format("Array/Exists", watch));
watch = Stopwatch.StartNew();
for (int rpt = 0; rpt < 10000; rpt++)
{
Array.IndexOf(arr, find[rpt]);
}
watch.Stop();
Console.WriteLine(format("Array/IndexOf", watch));
watch = Stopwatch.StartNew();
for (int rpt = 0; rpt < 10000; rpt++)
{
arr.Any(a => a == find[rpt]);
}
watch.Stop();
Console.WriteLine(format("Array/Any", watch));
watch = Stopwatch.StartNew();
for (int rpt = 0; rpt < 10000; rpt++)
{
set.Contains(find[rpt]);
}
watch.Stop();
Console.WriteLine(format("HashSet/Contains", watch));
}
结果
*******************************************
***** ContainsExistsAnyVeryShortRange *****
*******************************************
List/Contains: 1067 ticks
List/Exists: 2884 ticks
List/Any: 10520 ticks
Array/Contains: 1880 ticks
Array/Exists: 5526 ticks
Array/IndexOf: 601 ticks
Array/Any: 13295 ticks
HashSet/Contains: 6629 ticks
***************************************
***** ContainsExistsAnyShortRange *****
***************************************
List/Contains: 4ms
List/Exists: 28ms
List/Any: 138ms
Array/Contains: 6ms
Array/Exists: 34ms
Array/IndexOf: 3ms
Array/Any: 96ms
HashSet/Contains: 0ms
***************************************
********* ContainsExistsAny ***********
***************************************
List/Contains: 11504ms
List/Exists: 57084ms
List/Any: 257659ms
Array/Contains: 11643ms
Array/Exists: 52477ms
Array/IndexOf: 11741ms
Array/Any: 194184ms
HashSet/Contains: 3ms
编辑(2021年8月25日)
我添加了对于非常短的集合(10项)的比较,并且添加了Array.Contains
和Array.IndexOf
。你可以看到在这种小范围内,Array.IndexOf
是最快的。这是因为如@lucky-brian所说,n在这里非常小,因此for
-loop的性能优于稍微复杂的搜索算法。不过,我仍建议尽可能使用HashSet<T>
,因为它更好地反映了仅具有唯一值的意图,而对于小集合的差异低于1ms。
HashSet<T>
中不存在Any
方法,但它是IEnumerable<T>
的扩展方法,如果没有谓词,则时间复杂度为 O(1),如果有谓词,则时间复杂度为 O(n)。以下是源代码链接:https://github.com/Microsoft/referencesource/blob/master/System.Core/System/Linq/Enumerable.cs#L1288 - wertzuiArray.Exists(
https://learn.microsoft.com/en-us/dotnet/api/system.array.exists?view=netframework-4.7.2 - Mark Schultheiss