Solution.java

class Solution {
    private final int[] dirs = {-1, 0, 1, 0, -1};
    private int[][] grid1;
    private int[][] grid2;
    private int m;
    private int n;

    public int countSubIslands(int[][] grid1, int[][] grid2) {
        m = grid1.length;
        n = grid1[0].length;
        this.grid1 = grid1;
        this.grid2 = grid2;
        int ans = 0;
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (grid2[i][j] == 1) {
                    ans += dfs(i, j);
                }
            }
        }
        return ans;
    }

    private int dfs(int i, int j) {
        int ok = grid1[i][j];
        grid2[i][j] = 0;
        for (int k = 0; k < 4; ++k) {
            int x = i + dirs[k], y = j + dirs[k + 1];
            if (x >= 0 && x < m && y >= 0 && y < n && grid2[x][y] == 1) {
                ok &= dfs(x, y);
            }
        }
        return ok;
    }
}