200. Leetcode solution in java

class Solution {
    public int numIslands(char[][] grid) {
        if (grid == null || grid.length == 0 || grid[0].length == 0)
            return 0;
        int r = grid.length;
        int c = grid[0].length;
        int res = 0;
        for (int i = 0; i < r; i++) {
            for (int j = 0; j < c; j++) {
                if (grid[i][j] == '1') {
                    dfs(grid, i, j);
                    res++;
                }
            }
        }
        return res;
    }

    private void dfs(char[][] grid, int r, int c) {
        int rowNum = grid.length;
        int colNum = grid[0].length;
        if (r < 0 || r >= rowNum || c < 0 || c >= colNum || grid[r][c] == '0') {
            return;
        }
        grid[r][c] = '0';
        dfs(grid, r + 1, c);
        dfs(grid, r - 1, c);
        dfs(grid, r, c + 1);
        dfs(grid, r, c - 1);
    }
}