Number of Islands

题目描述：
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

分析：显然，这是一道dfs的题目，只需要对每一个”1”节点进行dfs最终得到有多少个联通块，即是解，其中，要注意vector为空的特判，如果，行已经为空，查看列的长度会运行出错。

代码如下：

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