PROBLEM LINK:

Setter-Misha Chorniy
Tester-Misha Chorniy
Editorialist-Abhishek Pandey

DIFFICULTY:

CHALLENGE

PRE-REQUISITES:

Varying. Challenge problem is usually an application of your skills in competitive programming in general.

PROBLEM:

Given an array $A[]$ of randomly generated sequences, we have to add some integer $D_i$ (need not be distinct) to each array element $A_i$, where $0\le D_i\le K$. Our goal is to maximize $\frac{1}{M}\sum_{i=1}^{M} B_i$ where $B_i=(A_1*A_2...*A_N)\%P_i$

QUICK ANALYSIS:

It seemed that contestants faced problems in getting a good solution. We're concluding that, because, Some of the trivial solutions got too high points than what they should have got. Majority of the contestants simply printed back the input, or used $input+rand()\%K$ &etc.

ANALYSIS:

The first thing I want to say is that, this editorial is incomplete. And it will remain so, until you guys dont contribute! Its impossible to have any hard and fast approach for the challenge problems, and for the editorial to be at its full potential, I want to request you guys to discuss your approach. Benefit in challenge problem is better gained by discussion, seeing flaws of your approach and analyzing other's strengths- and seeing what worked well, and to what degree. Hence, I would request the community to put forward their approach and intuition behind the question :).

As for editorial, I will try to discuss some of the approaches I saw, in both div1 and div2. I hope you people like it :)

1. Div2-

Not even 10 minutes passed from start of contest on the historical date of $6th$ $April,2018$ when Div2 had got the first few accepted solutions. I had a guess in mind, and I,curious as a doomed cat, decided to see what they did and confirm my intuition. And I dont know if it was fate, or destiny, or perhaps something else, but what I saw was an exact picture of what I had in my mind...

cin>> arr[i]; //Take array input. cout<< arr[i];//Print it back.

This approach got something $\approx 85-88$ points. It was $88.7$ when I checked last on $14th$.
Further solutions were also on similar lines. Eg-

cin>>arr[i]; cout<< arr[i]+rand()%k;

This one got $85.8$ points then. Sad luck for that guy :/

cin>>arr[i]; cout<< arr[i]+k;

This solution performed better, and got around$88-89$ points on average.

Some of the better solutions at div2 which got $\ge90$ involved-

Choose one of the primes. Lets call it $P$ Either largest, smallest, middle one or randomly any.
Make array ideal according to that prime, i.e. add $D_i$ so that $(A_1*A_2..*A_N)\%P$ is maximum.
Pray that this solution gets better points.

By roughly around 20-25 submissions, people experimented with what prime to take. Most of them settled on the median prime.

A good number of approaches used simulation and storing array and its result. Eg-

cin>>arr[i]; Store arr[i]+rand()%k;//Store in a vector etc. Compute score for the just stored array. Repeat above 250-400 times. Print the configuration with maximum score

Depending on luck and number of simulation, the above approach fetched $88-94.7$ points. I saw quite a few with $94.7$ points.

Some of the top approaches also use the concept of simulating till just about to time-out. The contestants chosed a distribution (random, or some other) which they simulated for $\approx3.8-3.95$ seconds where they sought to see which choice of $D_i$ is increasing score for a particular $A_i$. When about to time out, they aborted the process and printed the output they got.

2. Div1

The performance of Div1 was kind of similar to Div2 xD. One of the codes which got $91.1$ points at pretest was-

cin>>A[i]; cout<< A[i]+K/2;

Most of the approaches were common- like simulation till best answer, or take $rand()$ values $300-400$ times. Omitting the common approaches, the approaches of top solutions were quite distinct. (However, most of them are hard to decipher due to 300+ lines of code).

Some crucial optimizations were, however, seen. For example, lets say I got some values of $D_1,D_2...D_N$ and calculated the value of $Score=(A_1+D_1)*(A_2+D_2)*...*(A_N+D_N)\%P_i$. The top solutions preferred to change $D_i$ one by one, and re-calculate $Score$ in $O(Log(A_i+D_i))$ by using inverses as - $NewScore=({A_i+D_{old}})^{-1}*Score*(A_i+D_{new})\%P_i$. This method got $95+$ points on pretest.

Some of the good top codes deserve a mention here. These codes are what one can call crisp :)

As usual, I want to invite everybody (yes, everybody, not just the top scorers) to discuss their approaches so that we can have an engaging and learning insights into various intuitions :)

CHEF VIJJU'S CORNER:

1. The very first solution submitted by tester to test this problem was also simply printing back the input xD. After that as many as $16$ more submissions were made.

2.Lets discuss the setter's approach here. Lets take a random subset (serveral times) of array $P[]$ and multiply the primes in this subset. Lets call their multiple $MUL$. Now, we now that $MUL\%P_i=0$ where $P_i$ is a prime in the subset.Now, our aim is to get the value of $(A_1+D_1)*(A_2+D_2)....*(A_N+D_N)$ closer to (preferably exactly equal to) $MUL-x$. This is because, by applying $\%$ operation for various $P_i$, we will be left with $MUL\%P_i-x\%P_i=(-x)\%P_i$. We can go greedily and try to factorize $MUL-1,MUL-2,...,MUL-x$ (The exact value of $x$ can be experimented upon). The factorization will help us in manipulation of $D_i$ values.

One thing to check is that, $(A_1+K)*(A_2+K)..*(A_N+K)$ must have a value more than $MUL$ else the above scenario may not be possible. We can check this by using $log$ function, i.e. by changing expression $(A_1+K)*(A_2+K)..*(A_N+K)$
to-
$Log(A_1+K)+Log(A_2+K)+...+Log(A_N+K)$.

Similarly, $MUL$ can be expressed as $(Log(P_1) +Log(P_2)+....+Log(P_N))$

You can find good practice problem here

3.There is also a better random generator in C++ known as Mersenne Twister or mt19937. It is available from C++11 and later. Some of the advantages it has over rand() are that-

mt19937 has much longer period than that of rand. This means that it will take its random sequence much longer to repeat itself again.
It much better statistical behavior.
Several different random number generator engines can be initiated simultaneously with different seed, compared with the single “global” seed srand() provides