<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>

class Solution {
public:
    void getFlowMap(vector<vector<int>>& heights, set<pair<int, int>>& seen_tiles,
                    vector<vector<int>>& can_reach) {

        deque<pair<int, int>> flow_tiles(seen_tiles.begin(), seen_tiles.end());

        for (; !flow_tiles.empty(); flow_tiles.pop_front()) {
            pair<int, int> tile = flow_tiles.front();
            int row = tile.first;
            int col = tile.second;

            can_reach[row][col] += 1;

            for (vector<vector<int>> all_dels({{-1, 0}, {1, 0}, {0, -1}, {0, 1}}); vector<int> dels : all_dels ) {
                int new_row = row + dels[0];
                int new_col = col + dels[1];

                if (new_row >= 0 && new_row < heights.size() && new_col >= 0 &&
                    new_col < heights[0].size() &&
                    heights[new_row][new_col] >= heights[row][col] && seen_tiles.find(pair<int, int>{new_row, new_col}) == seen_tiles.end()) {
                        // cout<<"From ("<<row<<", "<<col<<") : "<<"Inserting tile ("<<new_row<<", "<<new_col<<")\n";
                        flow_tiles.push_back(pair<int, int>{new_row, new_col});
                        seen_tiles.insert(pair<int, int>{new_row, new_col});
                }
            }
        }
    }

    vector<vector<int>> pacificAtlantic(vector<vector<int>>& heights) {
        vector<vector<int>> can_reach(heights.size(),
                                      vector<int>(heights[0].size(), 0));
        set<pair<int, int>> flow_tiles;

        // Pacific Ocean
        for (int i = 0; i < heights.size(); i++) {
            flow_tiles.insert(pair<int, int>{i, 0});
        }
        for (int j = 0; j < heights[0].size(); j++) {
            flow_tiles.insert(pair<int, int>{0, j});
        }

        Solution::getFlowMap(heights, flow_tiles, can_reach);
        // Clean up
        flow_tiles.clear();

        // Atlantic Ocean
        // cout<<"\n\nAtlantic Ocean\n\n";
        for (int i = 0; i < heights.size(); i++) {
            flow_tiles.insert(
                pair<int, int>{i, static_cast<int>(heights[0].size() - 1)});
        }
        for (int j = 0; j < heights[0].size(); j++) {
            flow_tiles.insert(
                pair<int, int>{static_cast<int>(heights.size() - 1), j});
        }

        Solution::getFlowMap(heights, flow_tiles, can_reach);

        vector<vector<int>> ans;
        for (int i = 0; i < can_reach.size(); i++) {
            for (int j = 0; j < can_reach[0].size(); j++) {
                if(can_reach[i][j] == 2){
                    ans.push_back({i, j});
                }
            }
            // cout << "\n";
        }

        return ans;
    }
};
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