Thinking about it, it comes down to the level of analysis you choose. If you assume constant space per integer, then it is indeed solvable with O(1) additional

Thinking about it, it comes down to the level of analysis you choose. If you assume constant space per integer, then it is indeed solvable with O(1) additional space. For example, this is usually assumed when analyzing sorting algorithms and I guess this is also what the author had in mind.