/**
 * @param {character[][]} grid
 * @return {number}
 */
var numIslands = function(grid) {
    // simple case
    if (!grid || grid.length === 0) return 0;
    
    let m = grid.length;
    let n = grid[0].length;
    let islandCount = 0;
    
    // helper function BFS search on connected lands
    const removeLand = (r,c, grid) => {
        // four directions
        const directions = [[1,0],[-1,0],[0,1],[0,-1]];
        let q = [];
        q.push([r,c]);
        
        while (q.length !== 0) {
            let [cRow, cColumn] = q.shift();
            directions.forEach(([dy, dx]) => {
                dy += cRow;
                dx += cColumn;
                if (dy < 0 || dx < 0 || dy >= m || dx >= n || grid[dy][dx] !== "1") return;
                
                grid[dy][dx] = "#";
                q.push([dy, dx]);
            })
        }
    }
    
    for (let i = 0; i < m; i++) {
        for (let j = 0; j < n; j++) {
            if (grid[i][j] === "1") {
                grid[i][j] = "#";
                removeLand(i, j, grid);
                islandCount++;
            }
        }
    }
    
    return islandCount;
    
    
};