http://www.reddit.com/r/dailyprogrammer/comments/1rdtky/111113_challenge_142_easy_falling_sand/

gistfile1.cpp

#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <limits>
#include <string>
#include <vector>

using namespace std;

void PrintGrid(const vector<vector<char>>& grid) {
  for (int i = 0; i < grid.size(); ++i) {
    for (int j = 0; j < grid.size(); ++j) {
      cout << grid[i][j];
    }
    cout << endl;
  }
}

bool GridFall(vector<vector<char>>* grid) {
  bool changed = false;
  // Skip row above ground
  for (int i = grid->size() - 2; i >= 0; --i) {
    auto& current_row = grid->at(i);
    auto& next_row = grid->at(i + 1);
    for (int j = 0; j < grid->size(); ++j) {
      if (current_row[j] == '.' && next_row[j] == ' ') {
        changed = true;
        current_row[j] = ' ';
        next_row[j] = '.';
      }
    }
  }
  return changed;
}

int main() {
  int grid_size;
  cin >> grid_size;

  vector<vector<char>> grid(grid_size);
  char dummy_newline;
  cin.get(dummy_newline);
  cin >> noskipws;
  for (int i = 0; i < grid_size; ++i) {
    for (int j = 0; j < grid_size; ++j) {
      char grid_char;
      cin.get(grid_char);
      grid[i].push_back(grid_char);
    }
    cin.get(dummy_newline);
  }

  do {
    // Uncomment to view/debug transitory steps.
    //cout << "Intermediate grid state :- " << endl;
    //PrintGrid(grid);
  } while (GridFall(&grid));

  PrintGrid(grid);

  return 0;
}

/*
 * Input Description
 * On standard console input, you will be given an integer N which represents
 * the N x N grid of ASCII characters. This means there will be N-lines of
 * N-characters long. This is the starting grid of your simulated world: the
 * character ' ' (space) means an empty space, while '.' (dot) means sand, and
 * '#' (hash or pound) means stone. Once you parse this input, simulate the
 * world until all particles are settled (e.g. the sand has fallen and either
 * settled on the ground or on stone). "Ground" is defined as the solid surface
 * right below the last row.
 * Output Description
 * Print the end result of all particle positions using the input format for
 * particles.
 * Sample Inputs & Outputs
 * Sample Input
 * 5
 * .....
 *   #  
 *   #    
 *
 *       .
 * Sample Output
 *   .  
 * . #  
 * #    
 *     .
 *  . ..
 */