<< problem 66 - Diophantine equation Magic 5-gon ring - problem 68 >>

# Problem 67: Maximum path sum II

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 in triangle.txt (right click and 'Save Link/Target As...'), a 15K text file containing a triangle with one-hundred rows.

NOTE: This is a much more difficult version of Problem 18. It is not possible to try every route to solve this problem, as there are 299 altogether!
If you could check one trillion (10^12) routes every second it would take over twenty billion years to check them all. There is an efficient algorithm to solve it. ;o)

# My Algorithm

The algorithm and code were copied from problem 18:

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;

current[0] = 2 + last[0] = 3
current[1] = 3 + last[0] = 4
copy current to last (which becomes { 3, 4 })

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

# 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.

Input data (separated by spaces or newlines):
Note: Prepend the number of rows (100 for the original problem)

This is equivalent to
echo "" | ./67

Output:

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

# My code

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

The code contains #ifdefs 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;
unsigned int numRows = 100;

//#define ORIGINAL
#ifndef ORIGINAL
std::cin >> tests;
#endif

while (tests--)
{
#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];

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

# Hackerrank

My code solves 20 out of 20 test cases (score: 100%)

# Difficulty

Project Euler ranks this problem at 5% (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 rarely an option.

# 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
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The 310 solved problems (that's level 12) had an average difficulty of 32.6% at Project Euler and
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.

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