## 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"``;`
`}`