695. Max Area of Island

描述

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.)

Example 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]]
Given the above grid, return 6. Note the answer is not 11, because the island must be connected 4-directionally.
Example 2:
[[0,0,0,0,0,0,0,0]]
Given the above grid, return 0.

思路

用一个递归函数，统计周围的格子

代码

class Solution {
public:
    int maxAreaOfIsland(vector<vector<int>>& grid) {
        int max_area = 0;
        
        for(int i = 0; i<grid.size();i++)
        {
            for(int j = 0;j<grid[0].size();j++)
            {
                if(grid[i][j]==1)
                max_area = max(max_area,maxArea(grid,i,j));
            }
        }
     return max_area;
    }
    
    int maxArea(vector<vector<int>>& grid,int i,int j)
    {
        if(i>=0&&i<grid.size()&&j>=0&&j<grid[0].size()&&grid[i][j]==1)
        {
            grid[i][j] = 0;//这个条件很重要，没这个条件，递归结束不了了
            return 1+maxArea(grid,i-1,j)+maxArea(grid,i+1,j)+maxArea(grid,i,j-1)+maxArea(grid,i,j+1);
        }
        
        return 0;
    }
};