图论第4天

周末还是懈怠啊，不如有个随想录催促我，今天没咋干活，先把昨天的内容补上，

417. 太平洋大西洋水流问题

溯流而上是需要考虑的内容，其次就是只要他可以，就给他true，我要我四周，都比我高的继续找。比我高他就是true。

class Solution {
public:
    int neighbor[4][2] = {1,0,0,-1,-1,0,0,1};
    vector<vector<int>> pacificAtlantic(vector<vector<int>>& heights) {
        int n = heights.size();
        int m = heights[0].size();
        vector<vector<int>>result;
        vector<vector<bool>>Pacific_visited(n,vector<bool>(m,false));
        vector<vector<bool>>Atlantic_visited(n,vector<bool>(m,false));
        //横向
        for(int i = 0;i < n;i++){
            dfs(heights,Pacific_visited,i,0);
            dfs(heights,Atlantic_visited,i,m-1);
        }
        //纵向
        for(int j = 0;j < m;j++){
            dfs(heights,Pacific_visited,0,j);
            dfs(heights,Atlantic_visited,n-1,j);            
        }
        for(int i = 0;i < n;i++){
            for(int j = 0;j < m;j++){
                if(Pacific_visited[i][j] == true && Atlantic_visited[i][j] == true){
                    result.push_back({i,j});
                }
            }
        }
        return result;
    }

    void dfs(vector<vector<int>> &heights,vector<vector<bool>> &visited,int x,int y){
        if(visited[x][y] == true)return;
        visited[x][y] = true;
        for(int i = 0;i < 4;i++){
            int nextx = x + neighbor[i][0];
            int nexty = y + neighbor[i][1];
            if(nextx < 0 || nexty < 0 || nextx >= heights.size() || nexty >= heights[0].size())continue;
            if(heights[x][y] > heights[nextx][nexty])continue;
            dfs(heights,visited,nextx,nexty);
        }
        return;
    }
};

827.最大人工岛

卧槽卧槽，这题真有成就感啊，好难啊。

随想录帮忙解决了两件事：

1、isAllGrid的用法

2、这个岛屿是否标记过，visitedGrid

class Solution
{
public:
    int neighbor[4][2] = {1, 0, 0, -1, -1, 0, 0, 1};
    int count = 0;
    int index = 2;
    unordered_map<int, int> map;
    int largestIsland(vector<vector<int>> &grid)
    {
        int result = 0;
        int n = grid.size();
        int m = grid[0].size();
        vector<vector<int>> visited(n, vector<int>(m, 0));
        bool isAllGrid = true;
        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < m; j++)
            {
                if (grid[i][j] == 0)
                    isAllGrid = false;
                if (grid[i][j] == 1 && visited[i][j] == 0)
                {
                    count = 1;
                    visited[i][j] = index;
                    bfs(grid, visited, i, j);
                    map[index] = count;
                    // cout << index << "是" << map[index] << endl;
                    index++;
                }
            }
        }
        if (isAllGrid)
            return n * m;
        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < m; j++)
            {
                if (grid[i][j] == 0)
                {
                    result = max(result, findMax(grid, visited, i, j));
                    // cout << "grid[" << i << "][" << j << "] 结果是 " << result << endl;
                }
            }
        }
        // for (int i = 0; i < n; i++)
        // {
        //     for (int j = 0; j < m; j++)
        //     {
        //         cout << "visited[" << i << "][" << j << "]是" << visited[i][j] << endl;
        //     }
        // }
        // for (int i = 2; i <= index; i++)
        // {
        //     cout << "map[" << i << "]是" << map[i] << endl;
        // }

        return result;
    }
    void bfs(vector<vector<int>> &grid, vector<vector<int>> &visited, int x, int y)
    {
        visited[x][y] = index;
        for (int i = 0; i < 4; i++)
        {
            int nextx = x + neighbor[i][0];
            int nexty = y + neighbor[i][1];
            if (nextx < 0 || nexty < 0 || nextx >= grid.size() || nexty >= grid[0].size())
                continue;
            if (grid[nextx][nexty] == 1 && visited[nextx][nexty] == 0)
            {
                count++;
                bfs(grid, visited, nextx, nexty);
            }
        }
    }
    int findMax(vector<vector<int>> &grid, vector<vector<int>> &visited, int x, int y)
    {
        int sumMax = 1;
        unordered_set<int> visitedGrid;
        for (int i = 0; i < 4; i++)
        {
            int nextx = x + neighbor[i][0];
            int nexty = y + neighbor[i][1];
            if (nextx < 0 || nexty < 0 || nextx >= grid.size() || nexty >= grid[0].size())
                continue;
            if (visitedGrid.count(visited[nextx][nexty]))
                continue;
            if (grid[nextx][nexty] == 1)
            {
                sumMax += map[visited[nextx][nexty]];
                visitedGrid.insert(visited[nextx][nexty]);
            }
        }
        return sumMax;
    }
};

我的写法应该是有一些冗余，不过基本上可以覆盖思路。

1.首先先做遍历岛屿，标记所有岛屿的大小。

2.然后再遍历海洋，看哪块海洋连接在一起会让岛屿最大。

3.考虑加岛屿的时候要确认别重复加了。

4.考虑全岛屿情况。

5.在搞函数的时候，发生了for循环走不出来，visit没标记已经查过的岛屿的问题（这个查了一段时间才搞出来）

牛逼！睡觉！