查找多个数组中所有匹配元素的索引

Question

Suppose I have two arrays as follows

int[] first = { 1, 2, 3, 4, 5, 6, 12, 13, 14 };
int[] second = { 12, 13, 14, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9 };

I want the result as follows:

matching index from the first = 6,7,8 
matching index from second = 0,1,2 

Condition: I cannot sort the array to find the index and there can be any number of the array.

I am looking for some efficient solution and I will be glad for the help. Thanks in advance.

Below is the code I did for the two arrays:

class Program
{
    static void Main(string[] args)
    {
        int[] first = { 1, 2, 3, 4, 5, 6, 12, 13, 14 };
        int[] second = { 12, 13, 14, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9 };
        IndexArray sameIndexArray = CompareArray(first, second);
        Console.WriteLine("FOLLOWING ARE THE INDEX WITH SAME VALUE FOR FIRST ARRAY");
        foreach (var index in sameIndexArray.FirstArray)
        {
            Console.WriteLine(index);
        }
        Console.WriteLine("FOLLOWING ARE THE INDEX WITH SAME VALUE FOR SECOND ARRAY");
        foreach (var index in sameIndexArray.SecondArray)
        {
            Console.WriteLine(index);
        }

        Console.ReadKey();
    }

    private static IndexArray CompareArray(int[] firstArray, int[] secondArray)
    {
        IndexArray arrayIndex = new IndexArray();
        arrayIndex.FirstArray = new List<int>();
        arrayIndex.SecondArray = new List<int>();
        for (int i = 0; i < firstArray.Length; i++)
        {
            for (int j = 0; j < secondArray.Length; j++)
            {
                if (firstArray[i] == secondArray[j])
                {
                    arrayIndex.FirstArray.Add(i);
                    arrayIndex.SecondArray.Add(j);
                }
            }
        }

        return arrayIndex;
    }
}

public class IndexArray
{
    public List<int> FirstArray { get; set; }
    public List<int> SecondArray { get; set; }
}

Answer 1

Your solution is O(N^2). An O(N) or O(N log N) solution should be possible:

• Create a HashSet for each of the sets
• iterate over the first set, filtering by hashset2.Contains and print the indexes
• do the same vice versa

Something like this:

private static IndexArray CompareArray(int[] firstArray, int[] secondArray)
{
    IndexArray arrayIndex = new IndexArray();
    var hashset2 = new HashSet<int>(secondArray);
    for (int i = 0; i < firstArray.Length; i++)
    {
        if (hashset2.Contains(firstArray[i]))
            arrayIndex.FirstArray.Add(i);
    }
    var hashset1 = new HashSet<int>(firstArray);
    for (int i = 0; i < secondArray.Length; i++)
    {
        if (hashset1.Contains(secondArray[i]))
            arrayIndex.SecondArray.Add(i);
    }

    return arrayIndex;
}

Answer 2

If this is working code it might be a better fit on code review.

I would drop the

arrayIndex.FirstArray = new List<int>();
arrayIndex.SecondArray = new List<int>();

Add

public List<int> FirstArray  { get; } = new List<int>();
public List<int> SecondArray { get; } = new List<int>();

Arraylookup is fast but I would add

int first = firstArray[i];

And then use that.

WritelLine will write a line.