SpaceX Interview Code Overview and Analysis

Josh SulkinSep. 12, 2011

Problem Statement> ./indexer foo.txt bar.txt ./baz.txt

orange

apple

foo.txt

apple orangeorangeorange

bar.txt

apple

baz.txt

apple ./baz.txt 2 bar.txt 1 foo.txt 3orange bar.txt 1 2 3 foo.txt 1

Words, file names, and line numbers all sorted.

Runtime better than O(n2)

Proposed Solution

Step 1:

Step 2:

Worst-Case Runtime: O(n*k)n: number of words

k: length of longest word

• Sort the file names in place• Insert each word into a Trie

- the word is the key- the value is a vector of line numbers for each file

• Do a pre-order traversal of the Trie

- For each stored value print the key and the list of line numbers for each file

Trie

a

n

t

at

n

t

t

b

e yb

e y

Insert:• a• ant• at• be• by

• Pre-order traversal guarantees retrieval in lexicographical order• Insertion takes O(k) time (i.e. it’s independent of n)

Trie.h

Trie.h

MultiCount.h and SpaceXVector.h

SpaceXVector.h

Main.cpp

Modifications

Modifications

• Simplified interface to Trie• Made duplicate line number

detection more efficient– Used fact that line numbers are inserted

in order

• Added more input parameter checks– Throw exceptions if parameters are bad

(i.e. initializing MultiCount with negative numbers)

Runtime Tests

Use

r S

pace

Run

time

[s]

Number of Words (n) Length of Words (k)

k = 5 n = 1E+05