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

    public void sumOfTwoQualToTargetSum()
    {
        List list= new List();
        list.Add(1);
        list.Add(3);
        list.Add(5);
        list.Add(7);
        list.Add(9);

        int targetsum = 12;

        int[] arr = list.ToArray();

        for (int i = 0; i < arr.Length; i++)
        {
            for (int j = 0; j < arr.Length; j++)
            {
                if ((i != j) && ((arr[i] + arr[j]) == targetsum))
                {
                    Console.Write("i =" + i);
                    Console.WriteLine("j =" + j);
                }
            }
        }
    }