How to write an algorithm to check if the sum of any two numbers in an array/list matches a given number?

Question

How can I write an algorithm to check if the sum of any two numbers in an array/list matches a given number with a complexity of nlogn?

Answer 1

This one is O(n)

public static bool doesTargetExistsInList(int Target, int[] inputArray)
{
    if (inputArray != null && inputArray.Length > 0 )
    {
        Hashtable inputHashTable = new Hashtable();

        // This hash table will have all the items in the input array and how many times they appeard
        Hashtable duplicateItems = new Hashtable();

        foreach (int i in inputArray)
        {
            if (!inputHashTable.ContainsKey(i))
            {
                inputHashTable.Add(i, Target - i);
                duplicateItems.Add(i, 1);
            }
            else
            {
                duplicateItems[i] = (int)duplicateItems[i] + 1;    
            }

        }

        foreach (DictionaryEntry de in inputHashTable)
        {
            if ((int)de.Key == (int)de.Value)
            {
                if ((int)duplicateItems[de.Key] > 1)
                return true;
            }
            else if (inputHashTable.ContainsKey(de.Value))
            {
                return true;
            }
        }
    }
    return false;
}