I know that there are combinatoric libraries for this kind of thing. However, with me being new to C# and coding in general I found that I couldn't understand the code well enough to implement it in my solution.

The code below represents my solution to the problem in question. It works but I feel like I reinvinted the wheel on this one. Is there a better way of going about it than what I have?

Challenge Prompt

Given two integers: L and R,

find the maximal values of A xor B given, L ≤ A ≤ B ≤ R

Input Format The input contains two lines, L is present in the first line. R in the second line.

Constraints 1 ≤ L ≤ R ≤ 10³

Output Format The maximal value as mentioned in the problem statement.

Sample Input#00

 1
 10

Sample Output#00

15

10
15

Sample Output#01

7

In the second sample let's say L=10, R=15, then all pairs which comply to above condition are

10⊕10=0
10⊕11=1
10⊕12=6
10⊕13=7
10⊕14=4
10⊕15=5
11⊕11=0
11⊕12=7
11⊕13=6
11⊕14=5
11⊕15=4
12⊕12=0
12⊕13=1
12⊕14=2
12⊕15=3
13⊕13=0
13⊕14=3
13⊕15=2
14⊕14=0
14⊕15=1
15⊕15=0

Here two pairs (10,13) and (11,12) have maximum xor value 7 and this is the answer.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;


class Solution {

    public static List<int[]> Combinations( List<int> number_list ) 
    {
    
        // We are only choosing 2 values out of any list of numbers
        int[] TwoList = new int[2]; 

        List<int[]> result = new List<int[]>();

        List<int> stack = new List<int>(number_list);                    
                
        while (stack.Count > 0) 
        {

            int StackLast = stack.Count - 1;

            for (int i =0; i < stack.Count; i++) 
            {
                TwoList[0] = stack[StackLast];

                TwoList[1] = stack[i];

                result.Add(new int[] {TwoList[0], TwoList[1]});
            }
            stack.RemoveAt(StackLast);
        }
        
        return result;

    }

    static int MaxXor(int l, int r) 
    {
            
        List<int> NumList = new List<int>(Enumerable.Range(l, r - l + 1));

        // Inserts the list combinations as int arrays
        List<int[]> ComboList = Combinations(NumList); 

        int max = 0;

        foreach(var two in NumList)
        {
        
            int XorValue = two[0] ^ two[1];

            max = ( max > XorValue )? max : XorValue;
        }
    
        return max;
    }

    static void Main(String[] args) 
    {

        int _l;
        int _r;

        _l = Convert.ToInt32(Console.ReadLine());
         
        _r = Convert.ToInt32(Console.ReadLine());
    
        Console.WriteLine(MaxXor(_l, _r));
    }
}