给定一个包含了一些 0 和 1 的非空二维数组 grid 。
一个 岛屿 是由一些相邻的 1 (代表土地) 构成的组合，这里的「相邻」要求两个 1 必须在水平或者竖直方向上相邻。你可以假设 grid 的四个边缘都被 0（代表水）包围着。
找到给定的二维数组中最大的岛屿面积。(如果没有岛屿，则返回面积为 0 。)

示例 1:

[[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]]

对于上面这个给定矩阵应返回 6。注意答案不应该是 11 ，因为岛屿只能包含水平或垂直的四个方向的 1 。
示例 2:

[[0,0,0,0,0,0,0,0]]

对于上面这个给定的矩阵, 返回 0。

注意: 给定的矩阵grid 的长度和宽度都不超过 50。

class Solution {
public:
    int maxAreaOfIsland(vector<vector<int>>& grid) {
        R = grid.size();
        if(R == 0) return 0;
        C = grid[0].size();
        if(C == 0) return 0;
        visited = vector<bool>(R*C, false);
        this->grid = grid;
        int res = 0;

        for(int i = 0; i < R * C; i++){
            int x = i / C, y = i % C;

            if(grid[x][y] == 1 && !visited[i]){
                res = max(res, dfs(i));
            }
        }
        return res;
    }

    int dfs(int v){
        visited[v] = true;
        int res =  1;
        int x = v / C, y = v % C;
        // cout<<v<<endl;
        for(int i = 0; i< 4; i++){
            int nextX = x + dirs[i][0];
            int nextY = y + dirs[i][1];
            if(inArea(nextX, nextY) && grid[nextX][nextY] == 1 && !visited[nextX * C + nextY]){
                res += dfs(nextX * C + nextY);
            }
        }
        return res;

    }

    bool inArea(int x, int y){
        return x >=0 && x < R && y >= 0 && y < C;
    }

private:
    vector<vector<int>> grid;
    vector<bool> visited;
    int R;
    int C;
    int dirs[4][2]={{-1,0},{0, 1},{1, 0}, {0, -1}};
};

优化

class Solution {
public:
    int maxAreaOfIsland(vector<vector<int>>& grid) {
        R = grid.size();
        if(R == 0) return 0;
        C = grid[0].size();
        if(C == 0) return 0;
        visited = vector<bool>(R*C, false);
        this->grid = grid;
        int res = 0;

        for(int x = 0; x < R; x++){
            for(int y = 0; y < C; y++){
                if(grid[x][y] == 1 ){
                    res = max(res, dfs(x, y));
                }
            }
        }
        return res;
    }

    int dfs(int x, int y){
        if(x < 0 || x >= R || y < 0 || y >= C || grid[x][y] == 0) return 0;

        grid[x][y] = 0;
        int res = 1;
        res += dfs(x - 1, y);
        res += dfs(x, y + 1);
        res += dfs(x + 1, y);
        res += dfs(x, y - 1);
        return res;
    }

    bool inArea(int x, int y){
        return x >=0 && x < R && y >= 0 && y < C;
    }

private:
    vector<vector<int>> grid;
    vector<bool> visited;
    int R;
    int C;
    int dirs[4][2]={{-1,0},{0, 1},{1, 0}, {0, -1}};
};