using System;

public class Program

{

    public static void Main()

    {

        var input = new int[] { 3, 4, -1, 1 };

        Print(input);

        var res = FirstMissingPositiveNumber(input);

        Console.WriteLine("The missing number in array is " + res);

    }

    private static int FirstMissingPositiveNumber(int[] arr)

    {

        if(arr == null || arr.Length < 1)

            return 1;

        int n = arr.Length;

        int i = 0;

        while(i < n)

        {

            if(arr[i] >= 0 && arr[i] < n && arr[arr[i]] != arr[i])

            {

                Swap(arr, i, arr[i]);

            }

            else

                i++;

        }

        int k = 1;

        while(k < n && arr[k] == k)

            k++;

        if(n == 0 || k < n)

            return k;

        else

            return arr[0] == k ? k + 1 : k;

    }

    private static void Swap(int[] arr, int i, int j)

    {

        int temp = arr[i];

        arr[i] = arr[j];

        arr[j] = temp;

    }

    private static void Print(int[] arr)

    {

        Console.Write("Input Array is   ");

        Console.WriteLine("");

        foreach(var i in arr)

        {

            Console.Write(i + "  ");

        }

        Console.WriteLine("");

    }

}