Number of Islands leetcode

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:

11110
11010
11000
00000

Answer: 1

Example 2:

11000
11000
00100
00011

Answer: 3

每次遇见一个1 记录, 把和他同在一个island上的1都变为0, 这样继续遍历就能找到island的数目了

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