leetcode:Number of Islands

其实该题为求连通子图的个数，可以借助DFS的思想：

class Solution {
public:
    bool dfs(int i,int j,vector<vector<char>> &grid)
    {
       if(i>=0&&i<grid.size()&&j>=0&&j<grid[i].size()&&grid[i][j]==‘1‘)
        {
            grid[i][j] = ‘3‘;
            dfs(i-1,j,grid);
            dfs(i+1,j,grid);
            dfs(i,j-1,grid);
            dfs(i,j+1,grid);
            return true;
        }
        return false;
    }
    int numIslands(vector<vector<char>> &grid) {
        int num = 0;
        for(int i=0;i<grid.size();i++)
        {
            for(int j=0;j<grid[i].size();j++)
            {
                if(dfs(i,j,grid))
                    num++;
            }
        }
        return num;
    }
};