求矩陣中連續1的個數 Number of Islands

問題：

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

解決：

① 求島嶼數量的題的本質是求矩陣中連續區域的個數，採用DFS，基本思想是合併相鄰的lands，合併經過遞歸完成。時間複雜度爲O(m*n)。

class Solution { //6ms
    public int numIslands(char[][] grid) {
        if (grid == null || grid.length == 0 || grid[0].length == 0) return 0;
        int m = grid.length;
        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'){
                    count ++;
                    merge(grid,i,j);
                }
            }
        }
        return count;
    }
    public void merge(char[][] grid,int i,int j){
        int m = grid.length;
        int n = grid[0].length;
        if(i < 0 || i >= m || j < 0 || j >= n || grid[i][j] != '1') return;
        grid[i][j] = '*';
        merge(grid,i - 1,j);
        merge(grid,i + 1,j);
        merge(grid,i,j - 1);
        merge(grid,i,j + 1);
    }
}