storeCredit.cs

void Main()
{
	
	var linesInFile = File.ReadAllLines(@"C:\Temp\A-large-practice.in");
	var testCases = FileParser.Parse(linesInFile.Skip(1).ToArray());

	var sw = new Stopwatch();
	sw.Start();
	
	foreach (var testCase in testCases)
	{
		Solver.Solve(testCase);
		testCase.Print();
	}
	
	sw.Stop();
	Console.WriteLine(sw.ElapsedMilliseconds + "ms");
}

class Solver
{
	public static int[] FindResults(Item[] items, int credit)
	{
		var leftIndex = 0;
		var rightIndex = items.Length - 1;

		while (leftIndex < rightIndex)
		{
			var cost = items[leftIndex].Price + items[rightIndex].Price;
			if (cost == credit)
			{
				var a = items[leftIndex].OriginalIndex;
				var b = items[rightIndex].OriginalIndex;
				return new int[2] { Math.Min(a, b), Math.Max(a, b) };
			}
			else if (cost > credit)
			{
				rightIndex--;
			}
			else
			{
				leftIndex++;
			}
		}

		return new int[2];
	}
	
	public static void Solve(TestCase testCase)
	{
		var sortedItems = testCase.Items
			.Where(i => i.Price <= testCase.Credit)
			.OrderBy(i => i.Price)
			.ToArray();
		var result = FindResults(sortedItems, testCase.Credit);
		testCase.Solution = result;
	}
}

class FileParser
{
	public static TestCase[] Parse(string[] lines)
	{
		var testCases = new List<TestCase>();
		var counter = 1;
		while (lines.Length > 0)
		{
			var firstThreeLines = lines.Take(3).ToArray();
			var testCase = TestCaseParser.Parse(firstThreeLines);
			testCase.Index = counter++;
			testCases.Add(testCase);
			lines = lines.Skip(3).ToArray();
		}
		
		return testCases.ToArray();
	}
}

class TestCaseParser
{
	public static TestCase Parse(string[] lines)
	{
		var testCase = new TestCase();
		testCase.Credit = Int32.Parse(lines[0]);
		testCase.NumberOfItems = Int32.Parse(lines[1]);
		testCase.Items = ItemParser.Parse(lines[2]);
		return testCase;
	}
}

class ItemParser
{
	public static Item[] Parse(string line)
	{
		var prices = line.Split(' ');
		var items = new List<Item>();
		
		for (var i = 0; i < prices.Length; i++)
		{
			items.Add(new Item { OriginalIndex = i + 1, Price = Int32.Parse(prices[i])});
		}
		
		return items.ToArray();
	}
}

class TestCase
{
	public int Index { get; set;}
	public int Credit { get; set; }
	public int NumberOfItems { get; set; }
	public Item[] Items { get; set; }
	public int[] Solution { get; set; }
	public void Print()
	{
		if (Solution[0] == 0 && Solution[1] == 0)
		{
			Console.WriteLine("Cannot solve");
		}
		else
		{
			Console.WriteLine($"Case #{Index}: {Solution[0]} {Solution[1]}");
		}
	}
}

class Item
{
	public int OriginalIndex { get; set; }
	public int Price { get; set;}
}