Find pairs that sum to eight. O(n)

TwoIntegersThatSumToEight.cs

static void Main(string[] args)
{
    var input = new int[]{4, 5, 2, 3, 4, 1};
    var memory = new Dictionary<int, int>();
    var results = new List<(int idx1, int idx2)>();

    for (var i = 0; i < input.Length; i++)
    {
        if (memory.ContainsKey(input[i]))
            results.Add((memory[input[i]], i));
        else
            memory.Add(8 - input[i], i);
    }

    foreach(var (idx1, idx2) in results)
        Console.WriteLine($"Sum of {input[idx1]} and {input[idx2]} is 8");

}