200. Number of Islands

Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Example 1:

Answer: 1

Example 2:

Answer: 3

dfs

public class Solution {
    public int numIslands(char[][] grid) {
        int res = 0;
        for(int i = 0; i < grid.length; i++){
            for(int j = 0; j < grid[0].length ; j++){
                if(grid[i][j] == '1'){
                    dfs(grid, i, j);
                    res++;
                }
            }
        }
        return res;
    }
    public void dfs(char[][] grid, int x, int y){
        if( x < 0 || x > grid.length - 1 || y< 0 || y > grid[0].length - 1 || grid[x][y] == '0')
            return;
        grid[x][y] = '0';
        dfs(grid, x+1, y);
        dfs(grid, x-1, y);
        dfs(grid, x, y-1);
        dfs(grid, x, y+1);
    }

bfs

public class Solution {
    public int numIslands(char[][] grid) {
        int res = 0;
        Queue<int[]> queue = new LinkedList<>();
        for(int i = 0; i < grid.length; i++){
            for(int j = 0 ; j < grid[0].length; j++){
                if(grid[i][j] == '1'){
                    res++;
                    queue.add(new int[]{i,j});
                    while(!queue.isEmpty()){
                        int point[] = queue.poll();
                        int row = point[0];
                        int col = point[1];
                        if(grid[row][col] == '0') continue;
                        grid[row][col] = '0';
                        if(row > 0)
                            queue.add(new int[]{row-1,col});
                        if(col > 0 )
                            queue.add(new int[]{row, col-1});
                        if(row < grid.length -1)
                            queue.add(new int[]{row+1, col});
                        if(col < grid[0].length -1)
                            queue.add(new int[]{row, col+1});
                    }
                }
            }
        }
       
        return res;
    }
}