A question about your while loop : if the 1st condition 'i < args.Length -1' is false, then it won't check the second condition 'args[i+1] == args[i] + 1' ,is that right?
asking because at first glance it looks like the second condition would throw out of bounds error

Yeah, it does.

It is not actually brute force. It's O(n). Brute force here would be O(2^n) aka exponential.
And if you run your solution and his, you'll see, they both take around 50ms

Doesn't give the right answer for the large test?