C#: O(nlogn)
public static int pickingNumbers(List a) { if (a.Count <= 1) return a.Count;

a.Sort(); 
int result = 0;
int currentCount = 1;

for (int i = 0, j = 1; j < a.Count; j++)
{

    if (a[j] - a[i] <= 1)
    {
        currentCount++;
    }
    else
    {

        result = Math.Max(result, currentCount);
        currentCount = 1; 
        i = j; 
    }
}

result = Math.Max(result, currentCount);
return result;
}

}