Question

It’s an interview question:

There are 1 billion cell-phone numbers which has 11 digits, they are stored randomly in a file, for
example 12345678910, the first digit gotta be 1. Go through these numbers to see whether there is
one with duplicate, just see if duplicate exists, if duplicate found,
return True, or return False.
Only 10 MB memory allowed.

Here is my solution:

Hash all these numbers into 1000 files using hash(num)%1000, then the duplicates should fall into the same file.

After the hashing, I got 1000 small files, each of which contains 1 million numbers at most, right? I’m not sure about this, I simply do it 1 billion / 1000 = 1 million.

Then for each file, build a hash table to store each number and a flag representing its occurrence.

I guess, it will take 5 B to represent the number, 4 B for the lower 8 digits and 1 B for the upper 3 digits; and actually 1 bit will suffice the flag, because I just need to find out whether duplicate exists, only how many times. But how can I apply the 1 bit flag to each number? I’m stumbled, so I choose bool to be the flag, 1 B is taken.
So finally, each number in the hash table will take 5B<for number> + 1B<for flag> + 4B<for the next-pointer> = 10B, then each file will take 10M for the hash table.

That’s my stupid solution, Please give me a better one.

Thanks.

FOLLOW UP:

If there are no duplicates in these 1 billion phone numbers, given one
phone number, how to find out the given one is or is not in these 1
billion numbers? Use as few memory as possible.

I came up with 2 solutions,

1. The phone number can be represented using 5B as I said above, scan through the file, read one number a time, and xor the given number with the one read from the file, if the result is 0, then the given one is in the file, it’ll take O(n) time, right?

2. Partition these numbers into 2 small files according to the leading bit, which means, those numbers with a leading 1-bit go to a file, leading 0-bit go to another file, meanwhile count how many numbers in each file, if the given number fall into the 1-bit file and the 1-bit file’s count is not full, then again partition the 1-bit file according to the secondary leading-bit, and check the given number recursively; if the 1-bit file is full, then the given number gotta be in the file, it’ll take O(logn) time, right?

Answer 1

Fastest solution (also in terms of programmer overhead 🙂

# Generate some 'phones'
yes 1 | perl -wne 'chomp; ++$a; print $_."$a\n";' > phones.txt

# Split phones.txt in 10MB chunks
split -C 10000000 phones.txt

# Sort each 10MB chunk with 10MB of memory
for i in x??; do sort -S 10M $i > $i.srt; echo -ne "$i.srt\0" >> merge.txt; done

# Merge the shorted chunks with 10MB of memory
sort -S 10M --files0-from=merge.txt -m > sorted.txt

# See if there is any duplicates
test -z $(uniq -d merge.txt)

Check that the memory usage constraint is met with pmap $(pidof sort) for example: