Sorting an array in linear time

Question

I need to find a method to sort an array in $O(n)$ time complexity. I saw this link, however I'm not sure how to apply it to the elements I need.

Input: an array $A$ of length $n$, containing values from $1$ to $n^2$

Output: a sorted array $A$

Can someone explain in pseudocode or in words how to do this?