Hey! so you can keep a list of what numbers you need to increase the mex and if you have just one more you need, you can change a number into the number you need and you are done right? for example 0 1 2 3 4 5 100 => you want MEX 6, you have 0-5 so all you need is 6, so change the 100 to 6

but that relies on you having something to change. for example 0 1 2 3 4 5 => you want MEX 6. You have 0-5 so all you need is 6, but you can't change anything else to get a 6 because that means you lose one of the 1-5.

So, you have to check whether you have sufficient number of elements to get your mex+1. If you have < k elements in an array, you can't have MEX of k. So, you need to check if the number of elements is > mex+1, otherwise you can't achieve mex + 1.

Hope this helps!