跟求岛屿数量的题目差不多，依旧是深度搜索或者广度搜索问题 

class Solution {
private:
    int maxAreaOfIsland(vector<vector<int>>& grid,vector<vector<bool>>& visited,int x,int y){
        if(x<0||x>=grid.size()||y<0||y>=grid[0].size())
            return 0;
        else if(grid[x][y]==0||visited[x][y])
            return 0;
        else{
            visited[x][y]=true;
            return 1+maxAreaOfIsland(grid,visited,x,y+1)+maxAreaOfIsland(grid,visited,x+1,y)+maxAreaOfIsland(grid,visited,x,y-1)+maxAreaOfIsland(grid,visited,x-1,y);
        }
    }
public:
    int maxAreaOfIsland(vector<vector<int>>& grid) {
        int result=0;
        int m=grid.size(),n=grid[0].size();
        vector<vector<bool>> visited(m,vector<bool>(n,false));
        for(int i=0;i<m;i++)
            for(int j=0;j<n;j++)
                if(grid[i][j]==1&&!visited[i][j])
                    result=max(result,maxAreaOfIsland(grid,visited,i,j));
        return result;
    }
};