//DFS solution
//Time: O(m * n * output)
//Space: O(output) for call stack
class Solution {
    public int numIslands(char[][] grid) {
        int ans = 0;
        for(int i = 0; i < grid.length; i++) {
            for(int j = 0; j < grid[0].length; j++) {
                if(grid[i][j] == '1') {
                    ans++;
                    deleteIsland(grid, i, j);
                }
            }
        }
        return ans;
    }

    private void deleteIsland(char[][] grid, int i, int j) {
        if(i < 0 || j < 0 || i >= grid.length || j >= grid[0].length || grid[i][j] == '0') {
            return;
        }
        grid[i][j] = '0';
        deleteIsland(grid, i - 1, j);
        deleteIsland(grid, i, j - 1);
        deleteIsland(grid, i + 1, j);
        deleteIsland(grid, i, j + 1);
    }
}