How to sort an integer array into negative, zero, positive part without changing relative position?

Question

Give an O(n) algorithm which takes as input an array S, then divides S into three sets: negatives, zeros, and positives. Show how to implement this in place, that is, without a

Answer 1

The C++ library has a stable_partition algorithm which requires n comparisons and O(n log n) swaps when it runs in-place.

As @Ted points out, the problem requires two applications of this algorithm.