1905-count-sub-islands.js

var countSubIslands = function (grid1, grid2) {
    let ROWS = grid1.length,
        COLS = grid1[0].length;
    let visit = new Set();

    const dfs = function (r, c) {
        let flatCoord = r * COLS + c;
        if (
            r < 0 ||
            c < 0 ||
            r == ROWS ||
            c == COLS ||
            grid2[r][c] == 0 ||
            visit.has(flatCoord)
        )
            return true;

        visit.add(flatCoord);
        let res = true;
        if (grid1[r][c] == 0) res = false;

        res = dfs(r - 1, c) && res;
        res = dfs(r + 1, c) && res;
        res = dfs(r, c - 1) && res;
        res = dfs(r, c + 1) && res;
        return res;
    };

    let count = 0;
    for (let r = 0; r < ROWS; r++)
        for (let c = 0; c < COLS; c++)
            if (grid2[r][c] && !visit.has(r * COLS + c) && dfs(r, c))
                count += 1;
    return count;
};