200. Number of Islands
Medium
Given an m x n
2D binary grid grid
which represents a 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'
.
Solution:
class Solution {
/**
* @param String[][] $grid
* @return Integer
*/
function numIslands($grid) {
$islands = 0;
if ($grid != null && !empty($grid) && count($grid[0]) != 0) {
for ($i = 0; $i < count($grid); $i++) {
for ($j = 0; $j < count($grid[0]); $j++) {
if ($grid[$i][$j] == '1') {
$this>dfs($grid, $i, $j);
$islands++;
}
}
}
}
return $islands;
}
private function dfs(&$grid, $x, $y) {
if ($x < 0  count($grid) <= $x  $y < 0  count($grid[0]) <= $y  $grid[$x][$y] != '1') {
return;
}
$grid[$x][$y] = 'x';
$this>dfs($grid, $x + 1, $y);
$this>dfs($grid, $x  1, $y);
$this>dfs($grid, $x, $y + 1);
$this>dfs($grid, $x, $y  1);
}
}
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