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. Or just jump to my GitHub repository.

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 <queue>
       
#include <vector>
       
#include <iostream>
       
 
       
// 2D matrix: unfortunately x and y are swapped, so we need to write matrix[y][x]
       
// instead of the more common matrix[x][y]

       
typedef std::vector<std::vector<unsigned int>> Matrix;
       
 
       
// use a priority queue to find the next cell to process

       
struct Cell
       
{
       
  // position
       
  unsigned int x, y;
       
  // sum of shortest route so far
       
  unsigned long long weight;
       
 
       
  Cell(unsigned int x_, unsigned int y_, unsigned long long weight_)
       
  : x(x_), y(y_), weight(weight_) {}
       
 
       
  // std::priority_queue returns the LARGEST element, therefore I implement this function "the other way 'round"
       
  bool operator<(const Cell& cell) const
       
  {
       
      return weight > cell.weight; // ">" is not a typo !
       
  }
       
};
       
 
       
// breadth-search

       
unsigned long long search(const Matrix& matrix)
       
{
       
  // matrix height/width
       
  const auto size = matrix.size();
       
 
       
  // store already processed cells
       
  std::vector<std::vector<bool>> processed(matrix.size());
       
  for (auto& row : processed)
       
    row.resize(matrix.size(), false);
       
 
       
  // process cells in increasing distance from starting point
       
  std::priority_queue<Cell> next;
       
  // add starting point (upper left corner)
       
  next.push(Cell(0, 0, matrix[0][0]));
       
 
       
  while (!next.empty())
       
  {
       
    // get cell with the smallest weight
       
    Cell cell = next.top();
       
    // and remove it from the queue
       
    next.pop();
       
 
       
    // we have been here before ?
       
    // must have been on a shorter route, hence discard current cell
       
    if (processed[cell.y][cell.x])
       
      continue;
       
 
       
    processed[cell.y][cell.x] = true;
       
 
       
    // finished ?
       
    if (cell.x == size - 1 && cell.y == size - 1)
       
      return cell.weight;
       
 
       
    // one step right
       
    if (cell.x + 1 < size)
       
      next.push(Cell(cell.x + 1, cell.y, cell.weight + matrix[cell.y][cell.x + 1]));
       
    // one step down
       
    if (cell.y + 1 < size)
       
      next.push(Cell(cell.x, cell.y + 1, cell.weight + matrix[cell.y + 1][cell.x]));
       
    // one step up
       
    if (cell.y > 0)
       
      next.push(Cell(cell.x, cell.y - 1, cell.weight + matrix[cell.y - 1][cell.x]));
       
    // one step left
       
    if (cell.x > 0)
       
      next.push(Cell(cell.x - 1, cell.y, cell.weight + matrix[cell.y][cell.x - 1]));
       
  }
       
 
       
  return -1; // failed
       
}
       
 
       
int main()
       
{
       
  unsigned int size = 80;
       
//#define ORIGINAL

       
#ifndef ORIGINAL
       
  std::cin >> size;
       
#endif
       
 
       
  Matrix matrix(size);
       
  for (auto& row : matrix)
       
  {
       
    row.resize(size);
       
    for (auto& cell : row)
       
    {
       
#ifdef ORIGINAL
       
      // unfortunately, Project Euler used a CSV format which is a bit tricky to parse in C++
       
      cell = 0;
       
      // read until the number is complete or we run out of input
       
      while (std::cin)
       
      {
       
        char c;
       
        std::cin.get(c);
       
        // number complete ?
       
        if (c < '0' || c > '9')
       
          break;
       
 
       
        // add digit to current number
       
        cell *= 10;
       
        cell += c - '0';
       
      }
       
#else
       
      std::cin >> cell;
       
#endif
       
    }
       
  }
       
 
       
  // go !
       
  std::cout << search(matrix) << std::endl;
       
 
       
  return 0;
       
}

This solution contains 18 empty lines, 26 comments and 8 preprocessor commands.