# Why do I publish my solutions ?

Almost always the solutions for problems at Project Euler consist of two parts:

1. finding a mathematical way to break down the problem into its elements
2. writing an efficient program for step 1
I have to admit that I see step 2 as the easy part because I am a software engineer.

There are already several blogs or code repositories (vast majority hosted at GitHub) where you can find solutions to the Project Euler problems.

I strive to:

• explain my choice of algorithms and data structures
• write fully commented C++ source code that compiles without warnings and without any external libraries
• offer interactive tests for most problems
• link to relevant Wikipedia / MathWorld / OEIS pages
• link to other solutions, especially those that are written in other programming languages

Let's not forget that this website helps me, too: only if I can explain a solution to someone else then I can be sure that I truly understood it in the first place.
And practicing some of the lesser used features of C++ (such as algorithm hidden inside STL, like std::next_permutation) improves my overall coding skills as well.

Sounds like a win-win situation ... ☺

# "The Solutions On This Website Are Too Complicated ..."

"... someone else wrote much shorter/simpler programs !".
Yes, several solutions to the original Project Euler problems are indeed much more straightforward.
But they usually:

• don't scale well (see notes about Hackerrank)
• have about the same number of lines of code, but no comments
• use language-specific features, such as Python's BigNum support
And yes, sometimes my code is way too complicated, too.

# HackerRank

The commercial website HackerRank offers an online programming environment. They have a huge variety of coding exercises and competitions for beginners and "masters", too.

Some Project Euler problems are modified by HackerRank and in most cases they become (much) harder.
There is a strict time limit - often 2 seconds for C++ code - whereas Project Euler just recommends that your program should finish in less than 1 minute.

The majority of my programs have a perfect 100% score at HackerRank, unless indicated otherwise.
The input format of my interactive tests is usually identical to HackerRank's.

If HackerRank's modifications substantially change my algorithm then I need to insert #ifdef ORIGINAL, whereas ORIGINAL refers to the original Project Euler problem and #ifndef ORIGINAL (or just #else) contains HackerRank specific code.

Note: the test cases at Hackerrank "discovered" a few bugs in my code. Even though I had the correct result for Project Euler, some of my programs failed to pass all Hackerrank tests.
@Hackerrank: Thanks for including those nasty edge cases most of us forget about !

# Interactive Tests

I created this feature because I often had trouble in finding some hidden bugs in my code:
I had the right algorithm, hopefully the best data structure and somehow my results were incorrect.
If you use my interactive tests then you can hopefully find your bugs, too.

Please note that I set a execution timeout of 1 second on my server. Memory is typically limited to 64 MByte.
If too many users are running tests, then some processes might get killed in order to keep the server responsive.

In addition, some input/output values are forbidden - especially the default input from the original Project Euler problem.

Note: this feature is still under development and requires Javascript to be enabled.

# 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 already 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:

 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200
The 133 solved problems had an average difficulty of 16.9% at Project Euler and I scored 11,174 points (out of 12300) at Hackerrank's Project Euler+.
more about me can be found on my homepage.
some names mentioned on this site may be trademarks of their respective owners.
thanks to the KaTeX team for their great typesetting library !