[LeetCode-JAVA] Number of Islands

题目：

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

思路： 循环遍历数组，发现一个1就将其左右上下相连的所有1都找到 并且标记，同时计数器加1即可。

代码：

public class Solution {
    public int numIslands(char[][] grid) {
        int m = grid.length;
        if(m == 0) return 0;
        int n = grid[0].length;
        
        int count = 0;
        for(int i = 0 ; i < m ; i++){
            for(int j = 0 ; j < n ; j++){
                if(grid[i][j] == '1'){
                    replace(grid, i, j);
                    count++;
                }
            }
        }
        return count;
    }
    public void replace(char[][] grid, int i, int j){
        int m = grid.length;
        int n = grid[0].length;
        
        grid[i][j] = '#';
        if(j-1 >= 0 && grid[i][j-1] == '1')
            replace(grid, i, j-1);
        if(i-1 >= 0 && grid[i-1][j] == '1')
            replace(grid, i-1, j);
        if(i+1 < m && grid[i+1][j] == '1')
            replace(grid, i+1, j);
        if(j+1 < n && grid[i][j+1] == '1')
            replace(grid, i, j+1);
        
        return;
    }
}