Problem
https: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! 
Answer: 1074
Solution
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
static const char* test_data_fname = "./src/euler_18_test_data.txt";
static const char* data_fname = "./src/euler_18_data.txt";
int maximum_path_sum_1(const char* fname)
{
std::ifstream fin(fname);
if( !fin.is_open()){
std::cerr << "Failed to open input file: " << fname << std::endl;
return -1;
}
std::vector<std::vector<int> > lines;
for( std::string line ; std::getline(fin,line);){
std::stringstream ss(line);
std::string number;
std::vector<int> inner;
while (std::getline(ss,number,',')) {
inner.push_back(std::stoi(number));
}
lines.push_back(inner);
}
for( int i = lines.size()-1; i > 0 ; --i){
for( int j = 0 ; j < i; j++){
if( lines[i][j] > lines[i][j+1] ){
lines[i-1][j] += lines[i][j];
}else{
lines[i-1][j] += lines[i][j+1];
}
}
}
return lines[0][0];
}
#if ! defined UNITTEST_MODE
int main(int argc, char const *argv[])
{
std::cout << "Answer: " << maximum_path_sum_1(data_fname) << std::endl;
}
#endif // #if ! defined UNITTEST_MODE