External Sort

If you have a 20 GB file with one string per line, which sorting algorithm
would you use to sort the file and why?

We'll divide the file into chunks which are x megabytes each, where x is the amount of memory we have available. Each chunk is sorted separately and then saved back to the file system. Once all the chunks are sorted, we then merge the chunks(称为N路归并). At then end, we have a fully sorted file.

如何实现N路归并