Sort a book of short, flat, and long positions

Each account holds a position marker: -1 for short, 0 for flat, +1 for long. Rearrange the array in place so all shorts come first, then all flats, then all longs. Do it in one pass with constant extra space.

positions = [1, -1, 0, 1, -1]
-> [-1, -1, 0, 1, 1]

Sort the three-valued array in place in $O(n)$ time and $O(1)$ space.