Question

我们需要找到几个整数排序数组的交集。这是一个例子：

示例：

Input:
1,3,7,8
2,3,8,10
3,10,11,12,13,14

minSupport = 1

Output:

1 and 2: 2, 8
1 and 3: 3
2 and 3: 3, 10

我写了算法，它运行得很快。

    var minSupport = 2;
    var elementsCount = 10000;
    var random = new Random(123);

    // Numbers of each array are unique
    var sortedArrays = Enumerable.Range(0,elementsCount)
    .Select(x => Enumerable.Range(0,30).Select(t => random.Next(1000)).Distinct()
    .ToList()).ToList();
    var result = new List<int[]>();
    var resultIntersection = new List<List<int>>();


    foreach (var array in sortedArrays)
    {
        array.Sort();
    }



    var sw = Stopwatch.StartNew();

    //****MAIN PART*****//

    // This number(max value which array can contains) is known. 
    // Ofcourse we can use dictionary if donnt know maxValue
    var maxValue = 1000;

    var reverseIndexDict = new List<int>[maxValue];

    for (int i = 0; i < maxValue; i++)
    {
        reverseIndexDict[i] = new List<int>();
    }

    for (int i = 0; i < sortedArrays.Count; i++)
    {
        for (int j = 0; j < sortedArrays[i].Count; j++)
        {
            reverseIndexDict[sortedArrays[i][j]].Add(i);
        }
    }


    var resultMatrix = new List<int>[sortedArrays.Count,sortedArrays.Count];

    for (int i = 0; i < sortedArrays.Count; i++)
    {   
        for (int j = 0; j < sortedArrays[i].Count; j++)
        {
            var sortedArraysij = sortedArrays[i][j];

            for (int k = 0; k < reverseIndexDict[sortedArraysij].Count; k++)
            {
                if(resultMatrix[i,reverseIndexDict[sortedArraysij][k]]==null) resultMatrix[i,reverseIndexDict[sortedArraysij][k]] = new List<int>();

                resultMatrix[i,reverseIndexDict[sortedArraysij][k]].Add(sortedArraysij);    

            }
        }
    }


    //*****************//

    sw.Stop();

    Console.WriteLine(sw.Elapsed);

但是当元素数量大于10000时，我的代码会因outofmemoryException而崩溃。我如何改进算法或我可以做些什么来解决这个问题？

Answer 1

使用Distinct方法，如下所示：

...
var theDistinctListOfInts = new List<int>();
foreach(var listOfInts in theListsOfInts)
{
    theDistinctListOfInts = theDistinctListOfInts.Intersect(listOfInts);
}
...

Answer 2

如果您知道数组可以拥有的最大整数数，则可以执行以下操作：

var histoMatrix = new int[1000]; // the max number in arrays is 1000 here

for (int i = 0; i < sortedArrays.Count; i++)
{   
    for (int j = 0; j < sortedArrays[i].Count; j++)
    {
        var sortedArraysij = sortedArrays[i][j];

        histoMatrix[sortedArraysij]++;
    }
}

var resultMatrix = new List<int>();

for (int i = 0; i < 1000; i++)
{
    if (histoMatrix[i] == sortedArrays.Count)
        resultMatrix.Add(histoMatrix[i]);
}

在这种情况下，您甚至不需要对数组进行排序。

希望有所帮助

解决System.OutOfMemoryException

2 个答案: