[LeetCode] Number of Islands

This is a typical problem about searching. In fact, you can use either BFS or DFS for it. Personally, I use BFS because I think it is more intuitive and easy to implement.

The idea is fairly simple. We maintain another 2-dimensional vector visited to mark whether the corresponding position in grid has been visited and a variable count for the number of islands (initialized to be 0). Then we visit grid from left to right and top to bottom. Each time when we see a non-visited 1 in grid, we add the number of islands by 1 and mark all the connected 4-neighbors (BFS is applied here to get neighbors, neighbors of neighbors, neighbors of neighbors of neighbors, ...) of the non-visited 1 (including itself) as visited.

Note that a lot of the solutions in the discussion forum directly changes 1 in grid to 0 instead of using another 2-dimensional vector visited. However, I personally prefer to keep the input unchanged in a function with a return value.

The code is as follows.

class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        if (grid.empty()) return 0;
        int m = grid.size(), n = grid[0].size();
        vector<vector<bool> > visited(m, vector<bool> (n, false));
        int count = 0;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == '1' && !visited[i][j]){
                    count++;
                    markIslands(grid, visited, i, j);
                }
            }
        }
        return count;
    }
private:
    void markIslands(vector<vector<char> >& grid, vector<vector<bool> >& visited, int row, int col) {
        visited[row][col] = true;
        if (row - 1 >= 0 && grid[row - 1][col] == '1' && !visited[row - 1][col])
            markIslands(grid, visited, row - 1, col);
        if (row + 1 < grid.size() && grid[row + 1][col] == '1' && !visited[row + 1][col])
            markIslands(grid, visited, row + 1, col);
        if (col - 1 >= 0 && grid[row][col - 1] == '1' && !visited[row][col - 1])
            markIslands(grid, visited, row, col - 1);
        if (col + 1 < grid[0].size() && grid[row][col + 1] == '1' && !visited[row][col + 1])
            markIslands(grid, visited, row, col + 1);
    }
};