011.cpp

#include <stdio.h>
#include <vector>

std::vector<std::vector<int>> grid = {
  {  8, 2,22,97,38,15, 0,40, 0,75, 4, 5, 7,78,52,12,50,77,91, 8 },
  { 49,49,99,40,17,81,18,57,60,87,17,40,98,43,69,48, 4,56,62, 0 },
  { 81,49,31,73,55,79,14,29,93,71,40,67,53,88,30, 3,49,13,36,65 },
  { 52,70,95,23, 4,60,11,42,69,24,68,56, 1,32,56,71,37, 2,36,91 },
  { 22,31,16,71,51,67,63,89,41,92,36,54,22,40,40,28,66,33,13,80 },
  { 24,47,32,60,99, 3,45, 2,44,75,33,53,78,36,84,20,35,17,12,50 },
  { 32,98,81,28,64,23,67,10,26,38,40,67,59,54,70,66,18,38,64,70 },
  { 67,26,20,68, 2,62,12,20,95,63,94,39,63, 8,40,91,66,49,94,21 },
  { 24,55,58, 5,66,73,99,26,97,17,78,78,96,83,14,88,34,89,63,72 },
  { 21,36,23, 9,75, 0,76,44,20,45,35,14, 0,61,33,97,34,31,33,95 },
  { 78,17,53,28,22,75,31,67,15,94, 3,80, 4,62,16,14, 9,53,56,92 },
  { 16,39, 5,42,96,35,31,47,55,58,88,24, 0,17,54,24,36,29,85,57 },
  { 86,56, 0,48,35,71,89, 7, 5,44,44,37,44,60,21,58,51,54,17,58 },
  { 19,80,81,68, 5,94,47,69,28,73,92,13,86,52,17,77, 4,89,55,40 },
  {  4,52, 8,83,97,35,99,16, 7,97,57,32,16,26,26,79,33,27,98,66 },
  { 88,36,68,87,57,62,20,72, 3,46,33,67,46,55,12,32,63,93,53,69 },
  {  4,42,16,73,38,25,39,11,24,94,72,18, 8,46,29,32,40,62,76,36 },
  { 20,69,36,41,72,30,23,88,34,62,99,69,82,67,59,85,74, 4,36,16 },
  { 20,73,35,29,78,31,90, 1,74,31,49,71,48,86,81,16,23,57, 5,54 },
  {  1,70,54,71,83,51,54,69,16,92,33,48,61,43,52, 1,89,19,67,48 } };

int greatest_product(int length)
{
  int greatest = 0;

  for (int x = 0; x < grid.size(); x++) {
    for (int y = 0; y < grid[x].size(); y++) {
      for (int cx = -1; cx < 2; cx++) {
        for (int cy = -1; cy < 2; cy++) {
          int current = 1;

          for (int i = 0; i < length; i++) {
            if ((cx != 0 && cy != 0) &&
                x + cx * i >= 0 && x + cx * i < grid.size() &&
                y + cy * i >= 0 && y + cy * i < grid.size()) {
              current *= grid[x + cx * i][y + cy * i];
            } else {
              current = -1;
              break;
            }
          }

          if (current > greatest)
            greatest = current;
        }
      }
    }
  }

  return greatest;
}

int main(int argc,char** argv)
{
  printf("%i\n", greatest_product(4));

  return 0;
}