Give one of the threads (thread ID 0, for instance) special privileges. Its list is the one everything else is appended to, then there's only concatenation or copying if you have more than one thread.
Or, pre-allocate the memory and let each thread write to its own subset of the final collection and avoid the combine step entirely. This works regardless of the number of threads you use so long as you know the maximum amount of memory you might need to allocate. If it has no calculable upper bound, you will need to use other techniques.
Or, pre-allocate the memory and let each thread write to its own subset of the final collection and avoid the combine step entirely. This works regardless of the number of threads you use so long as you know the maximum amount of memory you might need to allocate. If it has no calculable upper bound, you will need to use other techniques.