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:
Input: 11110 11010 11000 00000 Output: 1
Example 2:
Input: 11000 11000 00100 00011 Output: 3
分析：
给定一个二维数组，其中由1(陆地)和0(水)组成，要求岛屿的数量，而岛屿则是陆地间上下左右相连后形成的，斜向的不算相连则构不成岛屿。
遍历整个数组，当元素为1时，岛屿数目加一，此时我们要把所有与这个位置相连的陆地全部变成水(1变成0)，然后继续遍历到结尾即可。
程序：
C++
class Solution {
public:
int numIslands(vector<vector<char>>& grid) {
if(grid.size() == 0 || grid[0].size() == 0)
return 0;
m = grid.size();
n = grid[0].size();
int res = 0;
for(int i = 0; i < m; ++i){
for(int j = 0; j < n; ++j){
if(grid[i][j] == '1'){
dfs(grid, i, j);
res++;
}
}
}
return res;
}
private:
void dfs(vector<vector<char>>& grid, int i, int j){
if(i < 0 || j < 0 || i >= m || j >= n || grid[i][j] == '0')
return;
grid[i][j] = '0';
dfs(grid, i-1, j);
dfs(grid, i, j-1);
dfs(grid, i+1, j);
dfs(grid, i, j+1);
}
int m;
int n;
};
Java
class Solution {
public int numIslands(char[][] grid) {
if(grid.length == 0 || grid[0].length == 0)
return 0;
m = grid.length;
n = grid[0].length;
int res = 0;
for(int i = 0; i < m; ++i){
for(int j = 0; j < n; ++j){
if(grid[i][j] == '1'){
dfs(grid, i, j);
res++;
}
}
}
return res;
}
private void dfs(char[][] grid, int i, int j){
if(i < 0 || j < 0 || i >= m || j >= n || grid[i][j] == '0')
return;
grid[i][j] = '0';
dfs(grid, i-1, j);
dfs(grid, i, j-1);
dfs(grid, i+1, j);
dfs(grid, i, j+1);
}
private int m;
private int n;
}
