函数在运行大量元素时无法通过性能测试:超出时间限制。 如何通过性能测试?
//Function finds indices of two elements whose sum is equal to as passed in the parameter
public static Tuple<int, int> FindTwoSum(IList<int> list, int sum)
{
for (int i = 0; i < list.Count; i++)
{
int sum2 = sum - list[i];
int index = list.IndexOf(sum2);
if (index > 0 )
{
return new Tuple<int, int>(i, index);
}
}
return null;
}
//Main function to call FindTwoSum method
public static void Main(string[] args)
{
Tuple<int, int> indices = FindTwoSum(new List<int>() { 1, 3, 5, 7, 9 },12);
Console.WriteLine(indices.Item1 + " " + indices.Item2);
}
最佳答案
从表面上看,散列解决方案应该是最快的 - 实际上,它可能适用于大小超过 2GB 的非常大的数组。
然而(令人惊讶的是)对于大小高达 50,000,000 个元素的 int
数组,对数组进行排序并使用适用于已排序数组的优化算法会更快。
这是一个可以在排序数组上使用的算法(请注意,它需要一个额外的数组,该数组仅用于在排序前指示元素的原始索引):
public static Tuple<int, int> FindTwoSumInSortedList(IList<int> list, int[] indices, int sum)
{
for (int i = 0, j = list.Count - 1; i < j;)
{
int s = list[i] + list[j];
if (s == sum)
return new Tuple<int, int>(indices[i], indices[j]);
else if (s < sum)
++i;
else
--j;
}
return null;
}
对原始列表进行排序需要一些额外的工作:
int n = 10000000;
int[] array = new int[n];
...
var indices = Enumerable.Range(0, n).ToArray();
Array.Sort(array, indices);
result = FindTwoSumInSortedList(array, indices, target);
这看起来确实需要大量的额外工作,但令我惊讶的是,它在包含 20,000,000 个元素的数组上的表现优于哈希算法。
我把我的测试程序贴在下面,以供批评。对于 FindTwoSumInSortedList()
算法,我已尝试使样本数据尽可能地难看。
我在 PC 上从 RELEASE 构建中获得的结果是:
n = 10,000,000
3031
(5000000, 5000001)
1292
(5000000, 5000001)
n = 20,000,000
6482
(10000000, 10000001)
2592
(10000000, 10000001)
n = 50,000,000
17408
(25000000, 25000001)
5653
(25000000, 25000001)
所以你可以看到排序算法的速度是原来的两倍多。这真让我吃惊!
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Runtime.InteropServices;
namespace ConsoleApplication1
{
class Program
{
public static void Main()
{
int n = 10000000;
int[] array = new int[n];
var rng = new Random(18789);
for (int i = 0; i < n; ++i)
array[i] = rng.Next(0, n);
array[n/2] = n;
array[n/2 + 1] = n+1;
var sw = Stopwatch.StartNew();
// This is too slow to test:
//var result = FindTwoSum(array, n*2+1);
//Console.WriteLine(sw.ElapsedMilliseconds);
//Console.WriteLine(result);
sw.Restart();
var result = FindTwoSumFaster(array, n*2 + 1);
Console.WriteLine(sw.ElapsedMilliseconds);
Console.WriteLine(result);
sw.Restart();
var indices = Enumerable.Range(0, n).ToArray();
Array.Sort(array, indices);
result = FindTwoSumInSortedList(array, indices, n*2+1);
Console.WriteLine(sw.ElapsedMilliseconds);
Console.WriteLine(result);
}
public static Tuple<int, int> FindTwoSum(IList<int> list, int sum)
{
for (int i = 0; i < list.Count; i++)
{
int sum2 = sum - list[i];
int index = list.IndexOf(sum2);
if (index > 0)
{
return new Tuple<int, int>(i, index);
}
}
return null;
}
public static Tuple<int, int> FindTwoSumInSortedList(IList<int> list, int[] indices, int sum)
{
for (int i = 0, j = list.Count - 1; i < j;)
{
int s = list[i] + list[j];
if (s == sum)
return new Tuple<int, int>(indices[i], indices[j]);
else if (s < sum)
++i;
else
--j;
}
return null;
}
public static Tuple<int, int> FindTwoSumFaster(IList<int> list, int sum)
{
if (list == null)
throw new NullReferenceException("Null list");
// constructing a hashset to have O(1) operations
var listSet = new HashSet<int>();
// number -> index mapping
// O(n) complexity
var listReverseSet = new Dictionary<int, int>();
int i = 0;
foreach (var elem in list)
{
if (!listSet.Contains(elem))
listSet.Add(elem);
listReverseSet[elem] = i++;
}
// O(n) complexity
int listCount = list.Count;
for (int index = 0; index < listCount; index++)
{
var elem = list[index];
if (listSet.Contains(sum - elem))
return new Tuple<int, int>(index, listReverseSet[sum - elem]);
}
return null;
}
}
}
关于c# - 使用大量元素的列表时超过时间限制?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/36293087/