Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 9 | Sorting and Searching
9.4 If you have a 2 GB file with one string per line, which sorting algorithm would you use
to sort the file and why?
SOLUTION
pg 66
When an interviewer gives a size limit of 2GB, it should tell you something - in this case, it
suggests that they don’t want you to bring all the data into memory.
So what do we do? We only bring part of the data into memory..
Algorithm:
How much memory do we have available? Let’s assume we have X MB of memory available.
1. Divide the file into K chunks, where X * K = 2 GB. Bring each chunk into memory and
sort the lines as usual using any O(n log n) algorithm. Save the lines back to the file.
2. Now bring the next chunk into memory and sort.
3. Once we’re done, merge them one by one.
The above algorithm is also known as external sort. Step 3 is known as N-way merge
The rationale behind using external sort is the size of data. Since the data is too huge and we
can’t bring it all into memory, we need to go for a disk based sorting algorithm.
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