NumberOfIsland.java

class Solution {

    private static final int[][] DIRS = new int[][] {
        new int[] {-1,0},
        new int[] {1,0},
        new int[] {0,-1},
        new int[] {0,1}
    };

    public int numIslands(char[][] grid) {
        int ans = 0;
        for (int r = 0; r < grid.length; r++) {
            for (int c = 0; c < grid[r].length; c++) {
                if (grid[r][c] == '1') {
                    ans++;
                    markWater(grid, r, c);
                }
            }
        }
        return ans;
    }

    private void markWater(char[][] grid, int r, int c) {
        if (isInbound(grid, r, c) && grid[r][c] != '0') {
            grid[r][c] = '0';
            for (var dir : DIRS) {
                markWater(grid, r + dir[0], c + dir[1]);
            }
        }
    }

    private boolean isInbound(char[][] grid, int r, int c) {
        return r >= 0 && r < grid.length && c >= 0 && c < grid[r].length;
    }
}