As H L Mencken used to say, “For every complex problem there is an answer that is clear, simple, and wrong.” Here, the problem looks simple but solving it efficiently needs a clever trick.

Hints: using a bitwise operator is advised.

There is an array of non-negative integers. A second array is formed by shuffling the elements of the first array and deleting a random element. Given these two arrays, find which element is missing in the second array.

The challenge is to write a small test program that will be as light as possible in terms of code and as fast as possible in terms of finding the solution. All the solutions will be ordered by number of characters used (=as light as possible) and performance (=as fast as possible) when run on a simple laptop (intel i5 2.6 Ghz and 8 GB RAM); 50% of the points will be attributed for each.

The code should look like the following:

// generate a list of random N Integers
int N = 1000;
List<Integer> first = generateList(N);
// shuffle it and remove one element
List<Integer> second = shuffleAnrRemoveElement(first);
// call a function returning the missing element
Integer missingElement = findMissingElement();
System.out.println("Missing element is " + missingElement);

N (generateList()’s argument) will be set to a small value to check the algorithm is producing the expected result, and then set to 33554431 (2^25 - 1).

Good luck! :-)

Assuming that you have a working Go development environment.

cd <this git repo root>
go install excelian/dhanszec/techChallenge/round3/multiCore
go install excelian/dhanszec/techChallenge/round3/singleCore
go install excelian/dhanszec/techChallenge/round3/multiCoreMin
go install excelian/dhanszec/techChallenge/round3/multiCoreXor
go install excelian/dhanszec/techChallenge/round3/singleCoreMin

# run multi-core (code is using multiple CPUs by default)
bin\multiCore.exe

# run multi-core with XOR operator (code is using multiple CPUs by default)
bin\multiCoreXor.exe

# run multi-core (minified) (code is using multiple CPUs by default)
bin\multiCoreMin.exe

# run single-core
bin\singleCore.exe

# run single-core (minified)
bin\singleCoreMin.exe

Tested on:

• i7-2600K 3.7GHz
• 8GB RAM
• Win7 64b

Multi core with XOR version: ~5,461ms (10 runs)