var numIslands = function(grid) {
    if (grid.length === 0) { return 0; }
    let count = 0;

    function depthSearch(x, y) {
        if (grid[x][y] === '1') {
            grid[x][y] = '0';
        } else {
            return;
        }

        if (x < grid.length - 1) {
            depthSearch(x+1, y);
        }

        if (y < grid[x].length - 1) {
            depthSearch(x, y+1);
        }

        if (x > 0 && x < grid.length) {
            depthSearch(x-1, y);
        }

        if (y > 0 && y < grid[x].length) {
            depthSearch(x, y-1);
        }
    }

    for (let i = 0; i < grid.length; i++) {
        for (let j = 0; j < grid[i].length; j++) {
            if (grid[i][j] === '1') {
                count++;
                depthSearch(i, j);
            }
        }
    }

    return count;
}