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:

11110
11010
11000
00000
Answer: 1

Example 2:

11000
11000
00100
00011
Answer: 3

遍历字符数组，如果遇到‘1’，计数器加1，并且通过DFS从当前点开始，对和当前点连通的‘1’进行标记，然后继续搜索数组，直到结束。代码如下：

public class Solution {
    public int numIslands(char[][] grid) {
        if(grid == null || grid.length == 0) return 0;
        int result = 0;
        for(int i = 0; i < grid.length; i++) {
            for(int j = 0; j < grid[0].length; j++) {
                if(grid[i][j] != '1') continue;
                result ++;
                expandIsland(i, j, grid);
            }
        }
        return result;
    }
    public void expandIsland(int i, int j, char[][] grid) {
        if(i < 0 || j < 0 || i == grid.length || j == grid[0].length) return;
        if(grid[i][j] == '1') {
            grid[i][j] = 'X';
            expandIsland(i - 1, j, grid);
            expandIsland(i + 1, j, grid);
            expandIsland(i, j + 1, grid);
            expandIsland(i, j - 1, grid);
        }
    }
}