Tuesday, August 16, 2016

Find shortest safe route in a path with landmines - GeeksforGeeks


Find shortest safe route in a path with landmines - GeeksforGeeks
Given a path in the form of a rectangular matrix having few landmines arbitrarily placed (marked as 0), calculate length of the shortest safe route possible from any cell in the first column to any cell in the last column of the matrix. We have to avoid landmines and their four adjacent cells (left, right, above and below) as they are also unsafe. We are allowed to move to only adjacent cells which are not landmines. i.e. the route cannot contains any diagonal moves.

The idea is to use Backtracking. We first mark all adjacent cells of the landmines as unsafe. Then for each safe cell of first column of the matrix, we move forward in all allowed directions and recursively checks if they leads to the destination or not. If destination is found, we update the value of shortest path else if none of the above solutions work we return false from our function.
// These arrays are used to get row and column
// numbers of 4 neighbours of a given cell
int rowNum[] = { -1, 0, 0, 1 };
int colNum[] = { 0, -1, 1, 0 };
 
// A function to check if a given cell (x, y)
// can be visited or not
bool isSafe(int mat[R][C], int visited[R][C],
            int x, int y)
{
    if (mat[x][y] == 0 || visited[x][y])
        return false;
 
    return true;
}
 
// A function to check if a given cell (x, y) is
// a valid cell or not
bool isValid(int x, int y)
{
    if (x < R && y < C && x >= 0 && y >= 0)
        return true;
 
    return false;
}
 
// A function to mark all adjacent cells of
// landmines as unsafe. Landmines are shown with
// number 0
void markUnsafeCells(int mat[R][C])
{
    for (int i = 0; i < R; i++)
    {
        for (int j = 0; j < C; j++)
        {
            // if a landmines is found
            if (mat[i][j] == 0)
            {
              // mark all adjacent cells
              for (int k = 0; k < 4; k++)
                if (isValid(i + rowNum[k], j + colNum[k]))
                    mat[i + rowNum[k]][j + colNum[k]] = -1;
            }
        }
    }
 
    // mark all found adjacent cells as unsafe
    for (int i = 0; i < R; i++)
    {
        for (int j = 0; j < C; j++)
        {
            if (mat[i][j] == -1)
                mat[i][j] = 0;
        }
    }
 
    // Uncomment below lines to print the path
    /*for (int i = 0; i < R; i++)
    {
        for (int j = 0; j < C; j++)
        {
            cout << std::setw(3) << mat[i][j];
        }
        cout << endl;
    }*/
}
 
// Function to find shortest safe Route in the
// matrix with landmines
// mat[][] - binary input matrix with safe cells marked as 1
// visited[][] - store info about cells already visited in
// current route
// (i, j) are cordinates of the current cell
// min_dist --> stores minimum cost of shortest path so far
// dist --> stores current path cost
void findShortestPathUtil(int mat[R][C], int visited[R][C],
                          int i, int j, int &min_dist, int dist)
{
    // if destination is reached
    if (j == C-1)
    {
        // update shortest path found so far
        min_dist = min(dist, min_dist);
        return;
    }
 
    // if current path cost exceeds minimum so far
    if (dist > min_dist)
        return;
 
    // include (i, j) in current path
    visited[i][j] = 1;
 
    // Recurse for all safe adjacent neighbours
    for (int k = 0; k < 4; k++)
    {
        if (isValid(i + rowNum[k], j + colNum[k]) &&
            isSafe(mat, visited, i + rowNum[k], j + colNum[k]))
        {
            findShortestPathUtil(mat, visited, i + rowNum[k],
                           j + colNum[k], min_dist, dist + 1);
        }
    }
 
    // Backtrack
    visited[i][j] = 0;
}
 
// A wrapper function over findshortestPathUtil()
void findShortestPath(int mat[R][C])
{
    // stores minimum cost of shortest path so far
    int min_dist = INT_MAX;
 
    // create a boolean matrix to store info about
    // cells already visited in current route
    int visited[R][C];
 
    // mark adjacent cells of landmines as unsafe
    markUnsafeCells(mat);
 
    // start from first column and take minimum
    for (int i = 0; i < R; i++)
    {
        // if path is safe from current cell
        if (mat[i][0] == 1)
        {
            // initailize visited to false
            memset(visited, 0, sizeof visited);
 
            // find shortest route from (i, 0) to any
            // cell of last column (x, C - 1) where
            // 0 <= x < R
            findShortestPathUtil(mat, visited, i, 0,
                                 min_dist, 0);
 
            // if min distance is already found
            if(min_dist == C - 1)
                break;
        }
    }
 
    // if destination can be reached
    if (min_dist != INT_MAX)
        cout << "Length of shortest safe route is "
             << min_dist;
 
    else // if the destination is not reachable
        cout << "Destination not reachable from "
             << "given source";
}
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