<< problem 17 - Number letter counts | Counting Sundays - problem 19 >> |
Problem 18: Maximum path sum I
(see projecteuler.net/problem=18)
By starting at the top of the triangle below and moving to adjacent numbers on the row below, the maximum total from top to bottom is 23.
3
7 4
2 4 6
8 5 9 3
That is, 3 + 7 + 4 + 9 = 23.
Find the maximum total from top to bottom of the triangle below:
75
95 64
17 47 82
18 35 87 10
20 04 82 47 65
19 01 23 75 03 34
88 02 77 73 07 63 67
99 65 04 28 06 16 70 92
41 41 26 56 83 40 80 70 33
41 48 72 33 47 32 37 16 94 29
53 71 44 65 25 43 91 52 97 51 14
70 11 33 28 77 73 17 78 39 68 17 57
91 71 52 38 17 14 91 43 58 50 27 29 48
63 66 04 68 89 53 67 30 73 16 69 87 40 31
04 62 98 27 23 09 70 98 73 93 38 53 60 04 23
NOTE: As there are only 16384 routes, it is possible to solve this problem by trying every route.
However, Problem 67, is the same challenge with a triangle containing one-hundred rows; it cannot be solved by brute force, and requires a clever method! ;o)
My Algorithm
The main idea is to build a data structure similar to the input data:
but instead of just storing the raw input we store the biggest sum up to this point.
All data is processed row-by-row
Of course, the first row consists of a single number and it has no "parents", that means no rows above it.
Therefore the "sum" is the number itself.
This row now becomes my "parent row" called last
.
For each element of the next rows I have to find its parents (some have one, some have two),
figure out which parent is bigger and then add the current input to it.
This sum is stored in current
.
When a row is fully processed, current
becomes last
.
When all rows are processed, the largest element in last
is the result of the algorithm.
Example:
1
2 3
4 5 6
initialize:
last[0] = 1;
read second line:
current[0] = 2 + last[0] = 3
current[1] = 3 + last[0] = 4
copy current to last (which becomes { 3, 4 })
read third line:
current[0] = 4 + last[0] = 7
current[1] = 5 + max(last[0], last[1]) = 9
current[2] = 6 + last[1] = 10
copy current to last (which becomes { 7, 9, 10 })
finally:
print max(last) = 10
Note
Exactly the same algorithm is used for problem 67.
Interactive test
You can submit your own input to my program and it will be instantly processed at my server:
This live test is based on the Hackerrank problem.
This is equivalent toecho "1 4 3 7 4 2 4 6 8 5 9 3" | ./18
Output:
(this interactive test is still under development, computations will be aborted after one second)
My code
… was written in C++ and can be compiled with G++, Clang++, Visual C++. You can download it, as well as the input data, 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>
#include <algorithm>
int main()
{
unsigned int tests = 1;
//#define ORIGINAL
#ifndef ORIGINAL
std::cin >> tests;
#endif
while (tests--)
{
unsigned int numRows = 15;
#ifndef ORIGINAL
std::cin >> numRows;
#endif
// process input row-by-row
// each time a number is read we add it to the two numbers above it
// choose the bigger sum and store it
// if all rows are finished, find the largest number in the last row
// read first line, just one number
std::vector<unsigned int> last(1);
std::cin >> last[0];
// read the remaining lines
for (unsigned int row = 1; row < numRows; row++)
{
// prepare array for new row
unsigned int numElements = row + 1;
std::vector<unsigned int> current;
// read all numbers of current row
for (unsigned int column = 0; column < numElements; column++)
{
unsigned int x;
std::cin >> x;
// find sum of elements in row above (going a half step to the left)
unsigned int leftParent = 0;
// only if left parent is available
if (column > 0)
leftParent = last[column - 1];
// find sum of elements in row above (going a half step to the right)
unsigned int rightParent = 0;
// only if right parent is available
if (column < last.size())
rightParent = last[column];
// add larger parent to current input
unsigned int sum = x + std::max(leftParent, rightParent);
// and store this sum
current.push_back(sum);
}
// row is finished, it become the "parent" row
last = current;
}
// find largest sum in final row
std::cout << *std::max_element(last.begin(), last.end()) << std::endl;
}
return 0;
}
This solution contains 13 empty lines, 17 comments and 7 preprocessor commands.
Benchmark
The correct solution to the original Project Euler problem was found in less than 0.01 seconds on an Intel® Core™ i7-2600K CPU @ 3.40GHz.
(compiled for x86_64 / Linux, GCC flags: -O3 -march=native -fno-exceptions -fno-rtti -std=gnu++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
February 24, 2017 submitted solution
April 3, 2017 added comments
Hackerrank
see https://www.hackerrank.com/contests/projecteuler/challenges/euler018
My code solves 6 out of 6 test cases (score: 100%)
Difficulty
Project Euler ranks this problem at 5% (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 rarely an option.
Links
projecteuler.net/thread=18 - the best forum on the subject (note: you have to submit the correct solution first)
Code in various languages:
C# www.mathblog.dk/project-euler-18/ (written by Kristian Edlund)
C github.com/eagletmt/project-euler-c/blob/master/10-19/problem18.c (written by eagletmt)
Java github.com/nayuki/Project-Euler-solutions/blob/master/java/p018.java (written by Nayuki)
Javascript github.com/dsernst/ProjectEuler/blob/master/18 Maximum path sum I.js (written by David Ernst)
Go github.com/frrad/project-euler/blob/master/golang/Problem018.go (written by Frederick Robinson)
Mathematica github.com/nayuki/Project-Euler-solutions/blob/master/mathematica/p018.mathematica (written by Nayuki)
Haskell github.com/nayuki/Project-Euler-solutions/blob/master/haskell/p018.hs (written by Nayuki)
Scala github.com/samskivert/euler-scala/blob/master/Euler018.scala (written by Michael Bayne)
Perl github.com/gustafe/projecteuler/blob/master/018-largest-sum-in-triangle.pl (written by Gustaf Erikson)
Those links are just an unordered selection of source code I found with a semi-automatic search script on Google/Bing/GitHub/whatever.
You will probably stumble upon better solutions when searching on your own.
Maybe not all linked resources produce the correct result and/or exceed time/memory limits.
Heatmap
Please click on a problem's number to open my solution to that problem:
green | solutions solve the original Project Euler problem and have a perfect score of 100% at Hackerrank, too | |
yellow | solutions score less than 100% at Hackerrank (but still solve the original problem easily) | |
gray | problems are already solved but I haven't published my solution yet | |
blue | solutions are relevant for Project Euler only: there wasn't a Hackerrank version of it (at the time I solved it) or it differed too much | |
orange | problems are solved but exceed the time limit of one minute or the memory limit of 256 MByte | |
red | problems are not solved yet but I wrote a simulation to approximate the result or verified at least the given example - usually I sketched a few ideas, too | |
black | problems are solved but access to the solution is blocked for a few days until the next problem is published | |
[new] | the flashing problem is the one I solved most recently |
I stopped working on Project Euler problems around the time they released 617.
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I scored 13526 points (out of 15700 possible points, top rank was 17 out of ≈60000 in August 2017) at Hackerrank's Project Euler+.
My username at Project Euler is stephanbrumme while it's stbrumme at Hackerrank.
Look at my progress and performance pages to get more details.
Copyright
I hope you enjoy my code and learn something - or give me feedback how I can improve my solutions.
All of my solutions can be used for any purpose and I am in no way liable for any damages caused.
You can even remove my name and claim it's yours. But then you shall burn in hell.
The problems and most of the problems' images were created by Project Euler.
Thanks for all their endless effort !!!
<< problem 17 - Number letter counts | Counting Sundays - problem 19 >> |