My task is the greedy algorithm:The activity selection problem is characteristic to this class of problems, where the goal is to pick the maximum number of activities that do not clash with each other.Sample input (the first line indicates the number of intervals; each subsequent line contains a start time and end time):52 41 76 99 115 8Corresponding output (maximum number of non-overlapping activities, followed by the indexes of those activities):31 5 4The output above indicates that the activities at time intervals 2-4, 5-8, and 9-11 can all be chosen with no overlap.The program works on small data, but it is too slow for really big input and I don't know how to improve it. Any ideas?  class Program{           static void Main(string[] args)    {       int N;       int[,] a;        using (StreamReader sr = new StreamReader(@prednasky.in))        {            string line;            line = sr.ReadLine();            N = int.Parse(line);            a = new int[N, 2];            int[] indx = new int[N];            int[] b = new int[N];            for (int i = 0; i < N; i++)            {                line = sr.ReadLine();                int[] nums = line.Trim().Split().Select(int.Parse).ToArray();                //the beginning of the interval to a[i, 0], indexes to a[i, 1]                a[i, 0] = nums[0];                a[i, 1] = i + 1;                //The end of the interval to b[i]                b[i] = nums[1];                //indx is array of indexes, i sort it instead of array a                indx[i] = i;            }            //now sort array of the end of the interval and array of indexes by it            Array.Sort(b, indx);            //now in 1 cycle going through a[index, *] and b. And save correct intervals to the start of a(becouse we already don't need it                     int last = 0;            int poc = 0;            for (int i = 0; i < N; i++)            {                if (a[indx[i], 0] > last)                {                                         a[indx[poc], 0] = a[indx[i], 1];                    poc++;                    last = b[i];                }            }            using (StreamWriter sr1 = new StreamWriter(@rozvrh.out))            {                sr1.WriteLine(poc);                for (int i = 0; i < poc; i++)                {                    sr1.Write(a[indx[i], 0]);                    sr1.Write( );                }                }        }    }}
C# greedy algorithm to pick activities that do not clash with each other (corrected)
c#;algorithm;programming challenge;time limit exceeded;interval