外部快排（External Quicksort）: 与普通快排不同的是，关键数据是一段buffer，首先将之前和之后的M/2个元素读入buffer并对该buffer中的这些元素进行排序，然后从被排序数组的开头（或者结尾）读入下一个元素。-External fast row (External Quicksort): fast row with common difference is that critical data is a buffer, before and after the first M/2 elements read into the buffer and the buffer in the order of these elements, and then was sorted array at the beginning (or end) read the next element.