【easy】695. Max Area of Island

题目：

Given a non-empty 2D array grid of 0's and 1's, an island is a group of 1's (representing land) connected 4-directionally (horizontal or vertical.) You may assume all four edges of the grid are surrounded by water.

Find the maximum area of an island in the given 2D array. (If there is no island, the maximum area is 0.)

[[0,0,1,0,0,0,0,1,0,0,0,0,0],
 [0,0,0,0,0,0,0,1,1,1,0,0,0],
 [0,1,1,0,1,0,0,0,0,0,0,0,0],
 [0,1,0,0,1,1,0,0,1,0,1,0,0],
 [0,1,0,0,1,1,0,0,1,1,1,0,0],
 [0,0,0,0,0,0,0,0,0,0,1,0,0],
 [0,0,0,0,0,0,0,1,1,1,0,0,0],
 [0,0,0,0,0,0,0,1,1,0,0,0,0]]
Given the above grid, return 6. Note the answer is not 11, because the island must be connected 4-directionally.

//参考了答案……思路有，但是dfs写的不对………………
class Solution {
public:
    int maxAreaOfIsland(vector<vector<int>>& grid) {
        int res = 0;
        for (int i = 0; i < grid.size(); i++)
        {
            for (int j = 0; j < grid[0].size(); j++)
            {
                if (grid[i][j])
                {
                    res = max(res, dfs(grid, i, j));
                }
            }
        }
        return res;
    }  

    int dfs(vector<vector<int>>& grid,int i,int j)
    {
        if (i < 0 || i >= grid.size() || j < 0 || j >= grid[0].size()) return 0;  //超出边界返回0
        if (grid[i][j])
        {
            grid[i][j] = 0;
            return  1 + dfs(grid, i, j + 1) + dfs(grid, i + 1, j)
                + dfs(grid, i, j - 1) + dfs(grid, i - 1, j);  //边界内，且为1，返回XXX
        }
        return 0;   //不是1 返回0
    }
};