Question

I am looking for a faster solution to the following problem.

I have three lists of products available in three different stores. I want to create a unique list containing all products available in the three stores and a unique list of products that appear in more than one store.

class Product{
    public int Id;
    // 
    public Product(int id)
    {
        this.Id = id;
    }
}
List<Product> store1 = new List<Product>();
List<Product> store2 = new List<Product>();
List<Product> store3 = new List<Product>();
List<Product> allUniqueProducts = new List<Product>();
List<Product> moreThanOneStore= new List<Product>();

Fill the lists with some arbitrary values

for(int i=0;i<10000;i++){
    store1.Add(new Product(i));
    store2.Add(new Product(i+2000));
    store3.Add(new Product(i+5000));
}

This is my solution however when the lists are large (in the region of 10,000) this code runs pretty slowly.

processStoreList(store1);
processStoreList(store2);
processStoreList(store3);

void processStoreList( List<Product> storeList ){

    foreach ( Product pd in storeList ){

        if ( !( allUniqueProducts.Count( x => x.Id == pd.Id ) > 0 ))
            allUniqueProducts.Add(pd);

        else if ( !( moreThanOneStore.Count( x => x.Id == pd.Id ) > 0 ))
            moreThanOneStore.Add(pd);
     }
}

Any suggestions?

Answer 1

You should use a Dictionary<int, Product> instead of List<Product>.

This way, ContainsKey will be O(1) instead of O(n)