<< problem 115 - Counting block combinations II | Red, green, and blue tiles - problem 117 >> |

# Problem 116: Red, green or blue tiles

(see projecteuler.net/problem=116)

A row of five black square tiles is to have a number of its tiles replaced with coloured oblong tiles chosen from red (length two), green (length three), or blue (length four).

If red tiles are chosen there are exactly seven ways this can be done.

If green tiles are chosen there are three ways.

And if blue tiles are chosen there are two ways.

Assuming that colours cannot be mixed there are 7 + 3 + 2 = 12 ways of replacing the black tiles in a row measuring five units in length.

How many different ways can the black tiles in a row measuring fifty units in length be replaced if colours cannot be mixed and at least one coloured tile must be used?

NOTE: This is related to Problem 117.

# Algorithm

The row is filling with an arbitrary sequence of black and colored blocks.

The length of the sequence is the sum of black and colored blocks.

For a known number of black and colored blocks, this is:

{black+colored}choose{black} = dfrac{(black+colored)!}{black!colored!}

It took me a little bit to write a `choose`

function such that the factorials don't overflow too soon.

## Modifications by HackerRank

As always, Hackerrank's input is ridiculously high and my poor C++ data types can't handle such numbers.

[TODO] find closed formula

# 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).

#include <iostream>
#include <vector>
#define ORIGINAL
// number of ways to choose n elements from k available

unsigned long long choose(unsigned long long n, unsigned long long k)
{
// n! / (n-k)!k!
unsigned long long result = 1;
// reduce overflow by dividing as soon as possible to keep numbers small
for (unsigned long long invK = 1; invK <= k; invK++)
{
result *= n;
result /= invK;
n--;
}
return result;
}
int main()
{
unsigned int tests;
std::cin >> tests;
while (tests--)
{
// length of row
unsigned int totalLength = 50;
std::cin >> totalLength;
unsigned long long sum = 0;
// try all four block lengths
for (unsigned int blockLength = 2; blockLength <= 4; blockLength++)
{
// maximum number of blocks of the current size
auto maxBlocks = totalLength / blockLength;
// insert 1 to maxBlocks colored blocks
for (unsigned int colored = 1; colored <= maxBlocks; colored++)
{
// remaining black tiles
auto black = totalLength - colored * blockLength;
// total number of black and colored tiles
auto tiles = black + colored;
// count combinations
auto combinations = choose(tiles, colored);
sum += combinations;
#ifndef ORIGINAL
// Hackerrank only
sum %= 1000000007;
#endif
}
}
std::cout << sum << std::endl;
}
return 0;
}

This solution contains 8 empty lines, 11 comments and 5 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 "1 7" | ./116`

Output:

*Note:* the original problem's input `50`

__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 less than 0.01 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 14, 2017 submitted solution

May 14, 2017 added comments

# Hackerrank

see https://www.hackerrank.com/contests/projecteuler/challenges/euler116

My code solves **1** out of **5** test cases (score: **0%**)

I failed **0** test cases due to wrong answers and **4** because of timeouts

# Difficulty

Project Euler ranks this problem at **30%** (out of 100%).

Hackerrank describes this problem as **medium**.

*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=116 - **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-116-coloured-tiles/ (written by Kristian Edlund)

Java: github.com/nayuki/Project-Euler-solutions/blob/master/java/p116.java (written by Nayuki)

Scala: github.com/samskivert/euler-scala/blob/master/Euler116.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.

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