Find first free element

Question

I looking for query which return first number not available in list

int[] list = new int[] { 1,4,2,5,6,7 };

For above example I expect to have result 3.

Answer 1

Perhaps something like this:

int result = Enumerable.Range(1, list.Length)
                       .Where(i => !list.Contains(i))
                       .FirstOrDefault();

This will return 0 if list contains all integers from 1 to n .

Answer 2

var first = Enumerable.Range(1, list.Max()).Except(list).First();