回想起来了，Cracking the coding interview里面有，但是三哥问的很细。
我回答的也基本上是can't bring all the data into memory, we need to divide the file into chunks which are 100 (equal to the array size), each chunk is sorted separately and then saved back to file system. Once all the chunks are sorted, we then merge the chunks, one by one.
四个数：b c e d f g
array : [a1, a2]
怎么执行？ 需要几步，或者多少次能排完？ 醉了