<p>There is an <code>m x n</code> rectangular island that borders both the <strong>Pacific Ocean</strong> and <strong>Atlantic Ocean</strong>. The <strong>Pacific Ocean</strong> touches the island&#39;s left and top edges, and the <strong>Atlantic Ocean</strong> touches the island&#39;s right and bottom edges.</p>

<p>The island is partitioned into a grid of square cells. You are given an <code>m x n</code> integer matrix <code>heights</code> where <code>heights[r][c]</code> represents the <strong>height above sea level</strong> of the cell at coordinate <code>(r, c)</code>.</p>

<p>The island receives a lot of rain, and the rain water can flow to neighboring cells directly north, south, east, and west if the neighboring cell&#39;s height is <strong>less than or equal to</strong> the current cell&#39;s height. Water can flow from any cell adjacent to an ocean into the ocean.</p>

<p>Return <em>a <strong>2D list</strong> of grid coordinates </em><code>result</code><em> where </em><code>result[i] = [r<sub>i</sub>, c<sub>i</sub>]</code><em> denotes that rain water can flow from cell </em><code>(r<sub>i</sub>, c<sub>i</sub>)</code><em> to <strong>both</strong> the Pacific and Atlantic oceans</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/06/08/waterflow-grid.jpg" style="width: 400px; height: 400px;" />
<pre>
<strong>Input:</strong> heights = [[1,2,2,3,5],[3,2,3,4,4],[2,4,5,3,1],[6,7,1,4,5],[5,1,1,2,4]]
<strong>Output:</strong> [[0,4],[1,3],[1,4],[2,2],[3,0],[3,1],[4,0]]
<strong>Explanation:</strong> The following cells can flow to the Pacific and Atlantic oceans, as shown below:
[0,4]: [0,4] -&gt; Pacific Ocean 
&nbsp;      [0,4] -&gt; Atlantic Ocean
[1,3]: [1,3] -&gt; [0,3] -&gt; Pacific Ocean 
&nbsp;      [1,3] -&gt; [1,4] -&gt; Atlantic Ocean
[1,4]: [1,4] -&gt; [1,3] -&gt; [0,3] -&gt; Pacific Ocean 
&nbsp;      [1,4] -&gt; Atlantic Ocean
[2,2]: [2,2] -&gt; [1,2] -&gt; [0,2] -&gt; Pacific Ocean 
&nbsp;      [2,2] -&gt; [2,3] -&gt; [2,4] -&gt; Atlantic Ocean
[3,0]: [3,0] -&gt; Pacific Ocean 
&nbsp;      [3,0] -&gt; [4,0] -&gt; Atlantic Ocean
[3,1]: [3,1] -&gt; [3,0] -&gt; Pacific Ocean 
&nbsp;      [3,1] -&gt; [4,1] -&gt; Atlantic Ocean
[4,0]: [4,0] -&gt; Pacific Ocean 
       [4,0] -&gt; Atlantic Ocean
Note that there are other possible paths for these cells to flow to the Pacific and Atlantic oceans.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> heights = [[1]]
<strong>Output:</strong> [[0,0]]
<strong>Explanation:</strong> The water can flow from the only cell to the Pacific and Atlantic oceans.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>m == heights.length</code></li>
	<li><code>n == heights[r].length</code></li>
	<li><code>1 &lt;= m, n &lt;= 200</code></li>
	<li><code>0 &lt;= heights[r][c] &lt;= 10<sup>5</sup></code></li>
</ul>

typedef vector<int> Point;

class Solution {
public:
    vector<vector<int>> pacificAtlantic(vector<vector<int>>& heights) {
        int rows = heights.size();
        int cols = heights[0].size();

        vector<Point> pacific_start;
        vector<Point> atlantic_start;
        
        for (int row = 0; row < rows; row++) {
            pacific_start.push_back(vector<int>{row, 0});
            atlantic_start.push_back(vector<int>{row, cols-1});
        }
        for (int col = 0; col < cols; col++) {
            pacific_start.push_back(vector<int>{0, col});
            atlantic_start.push_back(vector<int>{rows-1, col});            
        }

        set<Point> pacific = bfs(heights, pacific_start);
        set<Point> atlantic = bfs(heights, atlantic_start);

        vector<Point> intersection;

        for (set<Point>::iterator p = pacific.begin(); p != pacific.end(); p++) {
            if (atlantic.find(*p) != atlantic.end()) {
                intersection.push_back(*p);
            }
        }
        return intersection;
    }

private:
    set<Point> bfs(const vector<vector<int>>& grid, vector<Point>& start) {
        set<Point> seen;
        queue<Point> q;
        for (Point& p : start)
            q.push(p);
        while (!q.empty()) {
            Point p = q.front();
            q.pop();
            if (seen.find(p) != seen.end())
                continue;
            seen.insert(p);
            vector<Point> neighbours = get_neighbours(grid, p);
            for (Point& n : neighbours)
                q.push(n);
        }
        return seen;
    }

    vector<Point> get_neighbours(const vector<vector<int>>& grid, const Point& p) {
        vector<Point> neighbours;
        int rows = grid.size();
        int cols = grid[0].size();

        if (p[0]-1 >= 0 && grid[p[0]-1][p[1]] >= grid[p[0]][p[1]]) 
            neighbours.push_back(Point{p[0]-1, p[1]});
        if (p[0]+1 < rows && grid[p[0]+1][p[1]] >= grid[p[0]][p[1]]) 
            neighbours.push_back(Point{p[0]+1, p[1]});
        if (p[1]-1 >= 0 && grid[p[0]][p[1]-1] >= grid[p[0]][p[1]]) 
            neighbours.push_back(Point{p[0], p[1]-1});
        if (p[1]+1 < cols && grid[p[0]][p[1]+1] >= grid[p[0]][p[1]]) 
            neighbours.push_back(Point{p[0], p[1]+1});
        
        return neighbours;
    }
};
45200