using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var res = SubarraySum(new int[] {1, 2, 3}, 3);

        Console.WriteLine(res);

    }

    public static int SubarraySum(int[] nums, int k)

        {

            var dict = new Dictionary<int, int> { { 0, 1 } };

            var currentSum = 0;

            var count = 0;

            foreach (var num in nums)

            {

                currentSum += num;

                count += dict.GetValueOrDefault(currentSum - k, 0);

​

                if (!dict.TryAdd(currentSum, 1))

                {

                    dict[currentSum]++;

                }

            }

​

            return count;

        }

}