200. Number of Islands

200. 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

思路：感觉这个可以用来做消灭星星游戏的基本原理。如果发现一个元素是1，就把他周围所有的1像素变为0，而且具有连锁反应。因此，采用深度搜索dfs。

class Solution {    void dfs(vector<vector<char>> & grid, int x, int y)    {        int m=grid.size(),n=grid[0].size();        if(x<0||x>=m||y<0||y>=n||grid[x][y]=='0') return;        grid[x][y]='0';        dfs(grid,x-1,y);        dfs(grid,x+1,y);        dfs(grid,x,y-1);        dfs(grid,x,y+1);    }public:    int numIslands(vector<vector<char>>& grid) {       int cnt=0;       int m=grid.size();       if(!m) return 0;       int n=grid[0].size();       for(int i=0;i<m;i++)       for(int j=0;j<n;j++)       {           if(grid[i][j]=='1')           {               cnt++;               dfs(grid,i,j);           }       }       return cnt;    }};