@CS234_1716410234 thank you so much for your explanation, I wasn't able to crack this problem, but now did it, it's not an optimised code at all, it may have huge time complexity but it's working for the time being :

static int maxLength=0; static boolean status = false; static List array = new ArrayList<>(); static List array1 = new ArrayList<>();

public static int pickingNumbers(List<Integer> a) 
{
    Collections.sort(a);
    int min=0;

        for(int i=0; i<a.size(); i++)
        {
            min = a.get(i);
            if(i==0)
            {
                array1.add(min);                    
            }

            //System.out.println("New Minimum = " +min);

            for(int j=1; j<a.size(); j++)
            {
                if(a.get(j) == min+1)
                {
                    array1.add(a.get(j));
                    status=true;
                    //System.out.println("Adding array[j] > " +a.get(j));
                }

                else if(a.get(j) == min)
                {
                    array1.add(a.get(j));
                    status=true;
                    //System.out.println("Adding array[j] = " +a.get(j));
                }
            }

            if(status==true)
            {
                array.add(array1.size());
                array1.clear();
            }

            else if(status==false)
            {
                array1.clear();
                //System.out.println("Clearing....");
            }
        }

    //System.out.println(array1);

    if(array1.size()==1)
    {
        array1.clear();
    }

    for(int size: array)
    {
        if(size > maxLength)
        {
            maxLength = size;
        }
    }

    return maxLength;
}