<< problem 102 - Triangle containment | Pandigital Fibonacci ends - problem 104 >> |

# Problem 103: Special subset sums: optimum

(see projecteuler.net/problem=103)

Let S(A) represent the sum of elements in set A of size n. We shall call it a special sum set if for any two non-empty disjoint subsets,

B and C, the following properties are true:

i. S(B) != S(C); that is, sums of subsets cannot be equal.

ii. If B contains more elements than C then S(B) > S(C).

If S(A) is minimised for a given n, we shall call it an optimum special sum set.

The first five optimum special sum sets are given below.

n = 1: { 1 }

n = 2: { 1, 2 }

n = 3: { 2, 3, 4 }

n = 4: { 3, 5, 6, 7 }

n = 5: { 6, 9, 11, 12, 13 }

It seems that for a given optimum set, A = {a_1, a_2, ... , a_n}, the next optimum set is of the form B = {b, a_1+b, a_2+b, ... ,a_n+b}, where b is the "middle" element on the previous row.

By applying this "rule" we would expect the optimum set for n = 6 to be A = { 11, 17, 20, 22, 23, 24 }, with S(A) = 117.

However, this is not the optimum set, as we have merely applied an algorithm to provide a near optimum set.

The optimum set for n = 6 is A = { 11, 18, 19, 20, 22, 25 }, with S(A) = 115 and corresponding set string: `111819202225`

.

Given that A is an optimum special sum set for n = 7, find its set string.

NOTE: This problem is related to Problem 105 and Problem 106.

# Algorithm

`search`

recursively produces all ascending sequences where all elements are between `minElement`

and `maxElement`

.

`check`

returns `true`

if the set is "special" by analyzing all subsets:

each number is either part or not part of a subset. That can be encoding by a single bit which is 0 (no) or 1 (yes).

Then there are 2^size combinations → I just run a counter named `mask`

from `0000000`

to `1111111`

(actually, `0000000`

represents the empty set and can be skipped).

For each sum I encounter along the way, I set a bit in `sums`

. It must never be set twice (first rule).

Moreover, I track the lower and highest sum for each number of elements.

If the highest sum of all n-element subsets is higher than then lowest sum of (n+1)-subsets then rule 2 was violated.

All successfully verified sequences are stored in an ordered set `solution`

and the first one is printed.

## Modifications by HackerRank

My simple brute-force approach produces only time-outs.

There is a certain generating function I haven't fully figured out. All I see is that x[2n+1] = x[2n] and x[2m] = x[2m-1] - ?.

# My code

… was written in C++11 and can be compiled with G++, Clang++, Visual C++. You can download it, too.

The code contains `#ifdef`

s to switch between the original problem and the Hackerrank version.

Enable `#ifdef ORIGINAL`

to produce the result for the original problem (default setting for most problems).

#define ORIGINAL
#ifdef ORIGINAL
#include <iostream>
#include <vector>
#include <map>
typedef std::vector<unsigned int> Sequence;
std::map<unsigned int, Sequence> solutions;
// number of elements in full set

unsigned int finalSize;
// lower limit for the smallest element

unsigned int minElement;
// upper limit for the biggest element

unsigned int maxElement;
// return true if sequence is special

bool check(const Sequence& sequence)
{
// sum of all elements
unsigned int fullSum = 0;
for (auto x : sequence)
fullSum += x;
// mark each generated sum as true, no collisions allowed
std::vector<bool> sums(fullSum + 1, false);
// track the lowest and highest sum for each subset size
std::vector<unsigned int> maxSum(sequence.size() + 1, 0);
std::vector<unsigned int> minSum(sequence.size() + 1, fullSum + 1);
minSum[0] = maxSum[0] = 0; // empty set
unsigned int fullMask = (1 << sequence.size()) - 1;
// 2^elements iterations (actually, I ignore the empty set)
for (unsigned int mask = 1; mask <= fullMask; mask++)
{
unsigned int sum = 0;
unsigned int size = 0;
for (unsigned int element = 0; element < sequence.size(); element++)
{
// use that element ?
unsigned int bit = 1 << element;
if ((mask & bit) == 0)
continue;
sum += sequence[element];
// count subset size
size++;
}
// two subsets share the same sum ?
if (sums[sum])
return false;
sums[sum] = true;
// adjust lowest and highest sum of current subset
if (minSum[size] > sum)
minSum[size] = sum;
if (maxSum[size] < sum)
maxSum[size] = sum;
}
// make sure that no set will fewer elements has a higher sum
for (size_t i = 1; i < sequence.size(); i++)
if (maxSum[i] > minSum[i + 1])
return false;
// yes, have another solution
solutions[fullSum] = sequence;
return true;
}
// enumerate all sequences where each number is between minElement and maxElement and a_i > a_j if i > j

void search(Sequence& sequence) // manipulate in-place
{
// enough elements ? check whether it is "special"
if (sequence.size() == finalSize)
{
check(sequence);
return;
}
// all numbers which are bigger than the predecessor
auto last = sequence.empty() ? minElement - 1 : sequence.back(); // minus 1 because last always adds 1
for (unsigned int i = last + 1; i <= maxElement; i++)
{
sequence.push_back(i);
// go deeper ...
search(sequence);
sequence.pop_back();
}
}
int main()
{
// read set size
std::cin >> finalSize;
// crude heuristics for the lower and upper limits
minElement = 1;
maxElement = 10;
if (finalSize >= 5)
{
maxElement = finalSize * finalSize;
minElement = maxElement / 4;
}
Sequence empty;
search(empty);
// `n = 1: { 1 }`
// `n = 2: { 1, 2 }`
// `n = 3: { 2, 3, 4 }`
// `n = 4: { 3, 5, 6, 7 }`
// `n = 5: { 6, 9, 11, 12, 13 }`
// `n = 6: { 11, 18, 19, 20, 22, 25 }`
for (auto s : solutions)
{
for (auto x : s.second)
std::cout << x;
break;
}
return 0;
}
#else
// heavily modified Hackerrank problem

#include <iostream>
#include <vector>
typedef std::vector<unsigned int> Sequence;
// apply near-optimal algorithm to increase sequence by one element
// my algorithm builds the sequence in reverse

void next(Sequence& sequence) // modify in-place
{
auto middle = sequence[(sequence.size() - 1) / 2];
for (auto& x : sequence)
x = (x + middle) % 1000000007;
sequence.push_back(middle);
}
int main()
{
unsigned int length;
std::cin >> length;
Sequence sequence = { 1 };
sequence.reserve(length);
for (unsigned int i = 2; i <= length; i++)
next(sequence);
for (auto i = sequence.rbegin(); i != sequence.rend(); i++)
std::cout << *i << " ";
std::cout << std::endl;
return 0;
}
#endif

This solution contains 31 empty lines, 29 comments and 9 preprocessor commands.

# Interactive test

You can submit your own input to my program and it will be instantly processed at my server:

This is equivalent to`echo 6 | ./103`

Output:

*Note:* the original problem's input `7`

__cannot__ be entered

because just copying results is a soft skill reserved for idiots.

*(this interactive test is still under development, computations will be aborted after one second)*

# Benchmark

The correct solution to the original Project Euler problem was found in 4.27 seconds on a Intel® Core™ i7-2600K CPU @ 3.40GHz.

(compiled for x86_64 / Linux, GCC flags: `-O3 -march=native -fno-exceptions -fno-rtti -std=c++11 -DORIGINAL`

)

See here for a comparison of all solutions.

**Note:** interactive tests run on a weaker (=slower) computer. Some interactive tests are compiled without `-DORIGINAL`

.

# Changelog

May 16, 2017 submitted solution

May 16, 2017 added comments

# Hackerrank

see https://www.hackerrank.com/contests/projecteuler/challenges/euler103

My code solves **1** out of **31** test cases (score: **100%**)

I failed **0** test cases due to wrong answers and **30** because of timeouts

# Difficulty

Project Euler ranks this problem at **45%** (out of 100%).

Hackerrank describes this problem as **easy**.

*Note:*

Hackerrank has strict execution time limits (typically 2 seconds for C++ code) and often a much wider input range than the original problem.

In my opinion, Hackerrank's modified problems are usually a lot harder to solve. As a rule thumb: brute-force is never an option.

# Links

projecteuler.net/thread=103 - **the** best forum on the subject (*note:* you have to submit the correct solution first)

Code in various languages:

Python: www.mathblog.dk/project-euler-103-special-subset-sum/ (written by Kristian Edlund)

Scala: github.com/samskivert/euler-scala/blob/master/Euler103.scala (written by Michael Bayne)

# Heatmap

green problems solve the original Project Euler problem and have a perfect score of 100% at Hackerrank, too.

yellow problems score less than 100% at Hackerrank (but still solve the original problem).

gray problems are already solved but I haven't published my solution yet.

blue problems are solved and there wasn't a Hackerrank version of it at the time I solved it or I didn't care about it because it differed too much.

*Please click on a problem's number to open my solution to that problem:*

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My username at Project Euler is

**stephanbrumme**while it's stbrumme at Hackerrank.

<< problem 102 - Triangle containment | Pandigital Fibonacci ends - problem 104 >> |