Sorting an array of length $n$ in $O(m+n)$ time I need to find a method to sort an array in $O(m+n)$ time complexity.
I understand it should be a variation on Radix sort, however I am not familiar with it or its implementation...

Input: an array $A$ of length $n$, containing values from $-m$ to $m$
Output: a sorted array $A$

Any suggestions will be appreciated, thanks!
Use counting sort. In $O(n)$, count the number of elements of each value, and then in $O(m+n)$ go over the histogram and translate it to the sorted array.