[刷题] 200 Number of Islands

 1 class Solution {
 2 
 3 private:
 4     int d[4][2] = {{0,1},{1,0},{0,-1},{-1,0}};
 5     int m,n;
 6     vector<vector<bool>> visited;
 7     
 8     bool inArea( int x, int y){
 9         return x >= 0 && x < m && y >= 0 && y < n;
10     }    
11     
12     // 从grid[x][y]开始，进行floodfill 
13     // 保证(x,y)合法，且grid[x][y]是没有被访问过的陆地 
14     void dfs( vector<vector<char>>& grid, int x, int y ){
15         
16         visited[x][y] = true;
17         for( int i = 0 ; i < 4 ; i ++ ){
18             int newx = x + d[i][0];
19             int newy = y + d[i][1];
20             // if语句保证访问合法，不需再写递归终止条件 
21             if( inArea(newx, newy) && !visited[newx][newy] && grid[newx][newy] == ‘1‘)
22                 dfs( grid, newx, newy );
23         }
24         return;
25     }
26     
27 public:
28     int numIslands(vector<vector<char>>& grid) {
29         m = grid.size();
30         if( m == 0 )
31             return 0;
32         n = grid[0].size();
33         
34         visited = vector<vector<bool>>(m, vector<bool>(n,false));
35         
36         int res = 0;
37         for( int i = 0 ; i < m ; i ++ )
38             for( int j = 0 ; j < n ; j ++ )
39             if( grid[i][j] == ‘1‘ && !visited[i][j] ){
40                 res ++;
41                 dfs( grid, i, j );
42             }
43         return res;
44     }
45 };