C# Online Compiler | .NET Fiddle

<no name> by Anonymous

using System;
using System.Collections.Generic;
using System.Linq;
					
public class Program
{
	public static void Main()
	{
		var concreteArticles = new List<(int InternalNumber, int Id)> {
			new()
			{
				Id = 122,
				InternalNumber = 1
			},
			new()
			{
				Id = 1435,
				InternalNumber = 2
			},
			new()
			{
				Id = 76454,
				InternalNumber = 9
			},
			new()
			{
				Id = 456987,
				InternalNumber = 3
			},
			new()
			{
				Id = 3453,
				InternalNumber = 4
			}
		};
		
		Console.WriteLine(concreteArticles.Select(x => x.InternalNumber).FindNextAvailableInt());
	}
}

public static class EnumerableExtensions
{
	public static int FindNextAvailableInt(this IEnumerable<int> arr) => arr
		.OrderBy(i => i)
		.TakeWhile((i, index) => (index + 1) == i)
		.Count() + 1;	
}

​x
 
using System;
using System.Collections.Generic;
using System.Linq;
                    
public class Program
{
    public static void Main()
    {
        var concreteArticles = new List<(int InternalNumber, int Id)> {
            new()
            {
                Id = 122,
                InternalNumber = 1
            },
            new()
            {
                Id = 1435,
                InternalNumber = 2
            },
            new()
            {
                Id = 76454,
                InternalNumber = 9
            },
            new()
            {
                Id = 456987,
                InternalNumber = 3
            },
            new()
            {
                Id = 3453,
                InternalNumber = 4
            }
        };
        
        Console.WriteLine(concreteArticles.Select(x => x.InternalNumber).FindNextAvailableInt());
    }
}
​
public static class EnumerableExtensions
{
    public static int FindNextAvailableInt(this IEnumerable<int> arr) => arr
        .OrderBy(i => i)
        .TakeWhile((i, index) => (index + 1) == i)
        .Count() + 1;   
}

Node:1

->9,attitude:1,from:1

->11,attitude:1,from:9
->16,attitude:1,from:9

->6,attitude:1,from:11
->5,attitude:-1,from:11

->12,attitude:1,from:6
->5,attitude:-1,from:6

->19,attitude:1,from:5
->8,attitude:1,from:5
->14,attitude:1,from:5

Node:2

->1,attitude:1,from:2
->3,attitude:1,from:2

->9,attitude:1,from:1

->11,attitude:1,from:9
->16,attitude:1,from:9

->6,attitude:1,from:11
->5,attitude:-1,from:11

->12,attitude:1,from:6
->5,attitude:-1,from:6

->19,attitude:1,from:5
->8,attitude:1,from:5
->14,attitude:1,from:5

Node:3

->2,attitude:-1,from:3

->1,attitude:-1,from:2

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->6,attitude:-1,from:11
->5,attitude:1,from:11

->12,attitude:-1,from:6
->5,attitude:1,from:6

->19,attitude:-1,from:5
->8,attitude:-1,from:5
->14,attitude:-1,from:5

Node:4

->1,attitude:-1,from:4

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->6,attitude:-1,from:11
->5,attitude:1,from:11

->12,attitude:-1,from:6
->5,attitude:1,from:6

->19,attitude:-1,from:5
->8,attitude:-1,from:5
->14,attitude:-1,from:5

Node:5

->19,attitude:-1,from:5
->8,attitude:-1,from:5
->14,attitude:-1,from:5
->11,attitude:-1,from:5
->1,attitude:-1,from:5

->12,attitude:-1,from:19

->1,attitude:1,from:12

->9,attitude:1,from:1

->11,attitude:1,from:9
->16,attitude:1,from:9

->6,attitude:1,from:11

Node:6

->12,attitude:1,from:6
->5,attitude:-1,from:6

->1,attitude:-1,from:12

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->19,attitude:1,from:5
->8,attitude:1,from:5
->14,attitude:1,from:5

Node:7

->12,attitude:1,from:7
->6,attitude:1,from:7

->1,attitude:-1,from:12

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->6,attitude:-1,from:11
->5,attitude:1,from:11

->19,attitude:-1,from:5
->8,attitude:-1,from:5
->14,attitude:-1,from:5

Node:8

->6,attitude:1,from:8
->5,attitude:-1,from:8

->12,attitude:1,from:6
->5,attitude:-1,from:6

->1,attitude:-1,from:12

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->19,attitude:1,from:5

Node:9

->11,attitude:1,from:9
->16,attitude:1,from:9

->6,attitude:1,from:11
->5,attitude:-1,from:11

->12,attitude:1,from:6
->5,attitude:-1,from:6

->1,attitude:-1,from:12

->19,attitude:1,from:5
->8,attitude:1,from:5
->14,attitude:1,from:5

Node:10

->1,attitude:-1,from:10

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->6,attitude:-1,from:11
->5,attitude:1,from:11

->12,attitude:-1,from:6
->5,attitude:1,from:6

->19,attitude:-1,from:5
->8,attitude:-1,from:5
->14,attitude:-1,from:5

Node:11

->6,attitude:1,from:11
->5,attitude:-1,from:11

->12,attitude:1,from:6
->5,attitude:-1,from:6

->1,attitude:-1,from:12

->9,attitude:-1,from:1

->19,attitude:1,from:5
->8,attitude:1,from:5
->14,attitude:1,from:5

Node:12

->1,attitude:-1,from:12

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->6,attitude:-1,from:11
->5,attitude:1,from:11

->19,attitude:-1,from:5
->8,attitude:-1,from:5
->14,attitude:-1,from:5

Node:13

->1,attitude:-1,from:13

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->6,attitude:-1,from:11
->5,attitude:1,from:11

->12,attitude:-1,from:6
->5,attitude:1,from:6

->19,attitude:-1,from:5
->8,attitude:-1,from:5
->14,attitude:-1,from:5

Node:14

->5,attitude:-1,from:14

->19,attitude:1,from:5
->8,attitude:1,from:5

->12,attitude:1,from:19

->1,attitude:-1,from:12

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->6,attitude:-1,from:11

Node:15

->1,attitude:-1,from:15

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->6,attitude:-1,from:11
->5,attitude:1,from:11

->12,attitude:-1,from:6
->5,attitude:1,from:6

->19,attitude:-1,from:5
->8,attitude:-1,from:5
->14,attitude:-1,from:5

Node:16

->1,attitude:1,from:16

->9,attitude:1,from:1

->11,attitude:1,from:9

->6,attitude:1,from:11
->5,attitude:-1,from:11

->12,attitude:1,from:6
->5,attitude:-1,from:6

->19,attitude:1,from:5
->8,attitude:1,from:5
->14,attitude:1,from:5

Node:17

->5,attitude:-1,from:17
->12,attitude:1,from:17

->19,attitude:1,from:5
->8,attitude:1,from:5
->14,attitude:1,from:5
->11,attitude:1,from:5
->1,attitude:1,from:5

->12,attitude:1,from:19

->1,attitude:-1,from:12

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->6,attitude:-1,from:11

Node:18

->1,attitude:1,from:18

->9,attitude:1,from:1

->11,attitude:1,from:9
->16,attitude:1,from:9

->6,attitude:1,from:11
->5,attitude:-1,from:11

->12,attitude:1,from:6
->5,attitude:-1,from:6

->19,attitude:1,from:5
->8,attitude:1,from:5
->14,attitude:1,from:5

Node:19

->12,attitude:1,from:19
->5,attitude:-1,from:19

->1,attitude:-1,from:12

->9,attitude:-1,from:1

->11,attitude:-1,from:9
->16,attitude:-1,from:9

->6,attitude:-1,from:11
->5,attitude:1,from:11

Node:20

->1,attitude:1,from:20

->9,attitude:1,from:1

->11,attitude:1,from:9
->16,attitude:1,from:9

->6,attitude:1,from:11
->5,attitude:-1,from:11

->12,attitude:1,from:6
->5,attitude:-1,from:6

->19,attitude:1,from:5
->8,attitude:1,from:5
->14,attitude:1,from:5

Cached Result
Last Run:	8:56:49 pm
Compile:	0.126s
Execute:	0s
Memory:	208.30kb
CPU:	0.016s

View IL Code