C# greedy algorithm to pick activities that do not clash with each other

Question

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.

Input and output here:

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 int N; 
static int[,] a; 

static void Main(string[] args) 

{ 
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(w => int.Parse(w)).ToArray(); 

a[i, 0] = nums[0]; 
a[i, 1] = i + 1; 

b[i] = nums[1]; 

indx[i] = i; 
} 


Array.Sort(b, indx); 

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(" "); 
} 
} 
} 
} 
} 

Answer 1

Step 1 is to indent your code, because it's very difficult to read if it's not indented:

class Program 
{ 
    static int N; 
    static int[,] a; 

    static void Main(string[] args) 

    { 
        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(w => int.Parse(w)).ToArray(); 

                a[i, 0] = nums[0]; 
                a[i, 1] = i + 1; 

                b[i] = nums[1]; 

                indx[i] = i; 
            } 


            Array.Sort(b, indx); 

            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(" "); 
                } 
            } 
        } 
    } 
} 

A couple of comments:

• I don't see any reason for N and a to be static class members instead of local variables.
• I can't tell what the variable a is supposed to represent. Can it be given a more descriptive name?
• Your code doesn't have any comments explaining the algorithm you're implementing. Those would be helpful for understanding the code.