Can someone help me with the following problem?

We are given an integer $$$n$$$. There's an $$$ n \times n $$$ multiplication table as well.

We have to find the $$$MEX$$$ of the multiplication table. It can be easily solved in $$$ O({n}^2) $$$ but the constraints of the problem won't allow this. Therefore, can someone help me with an $$$ O(n) $$$ solution of this problem?