Check for missing number in sequence

Question

I have an List which contains 1,2,4,7,9 for example.

I have a range from 0 to 10.

Is there a way to determine what numbers are missin

Answer 1

This method does not use LINQ and works in general for any two IEnunumerable<T> where T :IComparable

public static IEnumerable<T> FindMissing<T>(IEnumerable<T> superset, IEnumerable<T> subset) where T : IComparable
{
    bool include = true;
    foreach (var i in superset)
    {
        foreach (var j in subset)
        {
            include = i.CompareTo(j) == 0;
            if (include)
                break;
        }
        if (!include)
            yield return i;
    }
}

Answer 2

for a List L a general solution (works in all programming languages) would be simply

L.Count()*(L.Count()+1)/2 - L.Sum();

which returns the expected sum of series minus the actual series.

for a List of size n the missing number is:

n(n+1)/2 - (sum of list numbers)