using System;

using System.Collections.Generic;

using System.Linq;

public static class ArrayHelper {

    public static IList<int> Intersection(int[] arr1, int[] arr2) {

        if(arr1 == null || arr1.Length == 0 || arr2 == null || arr2.Length == 0) {

            return new int[0];

        }

        int i = 0;

        int j = 0;

        var res = new List<int>();

        while(i < arr1.Length && j < arr2.Length) {

            if(arr1[i] == arr2[j]) {

                if(res.LastOrDefault() != arr1[i]) {

                    res.Add(arr1[i]);

                }

                i++;

                j++;

            } else if(arr1[i] < arr2[j]) {

                i++;

            } else {

                j++;

            }

        }

        return res;

    }

    public static int[] SlidingSum(int[] arr, int k) {

        if(k <= 0)

            throw new ArgumentException(nameof(k));

        if(arr == null)

            throw new ArgumentException(nameof(arr));

        var sum = arr.Take(Math.Min(arr.Length, k)).Sum();

        var res = new int[arr.Length];

        res[0] = sum;

        int right = k;

        for(int left = 1; left < res.Length; left++) {

            sum -= arr[left - 1];

            if(right < arr.Length) {

                sum += arr[right++];

            }            

            res[left] = sum;

        }

        return res;

    }

}

​

public class Program

{

    public static void Main()

    {

        var res = ArrayHelper.SlidingSum(new [] { 1, 3 ,5 ,3 , 4}, 4);

        Console.WriteLine("{0}", string.Join(",", res));

    }

}