200| Number of Islands

題目：

Given an m x n 2d grid map of '1’s (land) and '0’s (water), return 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:

Input: grid = [

[“1”,“1”,“1”,“1”,“0”],

[“1”,“1”,“0”,“1”,“0”],

[“1”,“1”,“0”,“0”,“0”],

[“0”,“0”,“0”,“0”,“0”]

]

Output: 1

Example 2:

Input: grid = [

[“1”,“1”,“0”,“0”,“0”],

[“1”,“1”,“0”,“0”,“0”],

[“0”,“0”,“1”,“0”,“0”],

[“0”,“0”,“0”,“1”,“1”]

]

Output: 3

Constraints:

m == grid.length

n == grid[i].length

1 <= m, n <= 300

grid[i][j] is ‘0’ or ‘1’.

class Solution {
    public int numIslands(char[][] grid) {

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

    public void callBFS(char[][] grid, int i,int j){
        if(i<0||i>=grid.length||j<0||j>=grid[i].length||grid[i][j]=='0')
        return;   
     grid[i][j]='0';
        callBFS(grid,i+1,j);//up
        callBFS(grid,i-1,j);//down
        callBFS(grid,i,j-1);//left
        callBFS(grid,i,j+1);//right

    }
}