为什么80%的码农都做不了架构师？>>> 
问题：
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
解决：
① 求岛屿数量的题的本质是求矩阵中连续区域的个数，采用DFS，基本思想是合并相邻的lands，合并通过递归完成。时间复杂度为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);
}
}