<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:
    bool inValidCoord(int i, int j,  int m, int n){
        return  (i >=0 && i<m) && ( j >=0 && j < n);
    }

    void dfsFunc(int i, int j, int m, int n, set<pair<int,int>>& visit, vector<vector<int>>& heights){
        stack<pair<int,int>> dfsStack;
        dfsStack.push({i,j});
        visit.insert({i, j});
        vector<pair<int, int>> steps={
                {0,1},
                {0, -1},
                {1, 0},
                {-1, 0}
            };

        while(dfsStack.size() > 0){
            pair<int, int> top = dfsStack.top();
            dfsStack.pop();

            for(int k = 0; k < 4; k++){
                pair<int, int> next = {top.first + steps[k].first, top.second + steps[k].second};

                if(inValidCoord(next.first, next.second, m, n) && visit.count({next.first,next.second}) == 0 && heights[top.first][top.second] <= heights[next.first][next.second]){
                            visit.insert({next.first, next.second});
                            dfsStack.push(next);
                }
            }
        }
    }

    vector<vector<int>> pacificAtlantic(vector<vector<int>>& heights) {
        int m = heights.size();
        int n = heights[0].size();

        vector<vector<int>>res;

        set<pair<int,int>> pacificVisit;
        set<pair<int,int>> atlanticVisit;
        for(int i= 0; i < m; i++){
            pair<int,int> pacificRoot{i, 0};
            pair<int,int> atlanticRoot{i, n -1};
            dfsFunc(pacificRoot.first, pacificRoot.second,m, n,  pacificVisit, heights);
            dfsFunc(atlanticRoot.first, atlanticRoot.second, m, n, atlanticVisit, heights);
        }

        for(int i= 0; i < n; i++){
            pair<int,int> pacificRoot{0, i};
            pair<int,int> atlanticRoot{m-1, i};
            dfsFunc(pacificRoot.first, pacificRoot.second, m, n,  pacificVisit, heights);
            dfsFunc(atlanticRoot.first, atlanticRoot.second,m, n,  atlanticVisit, heights);
        }

        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                pair<int,int> point{i, j};
                
                if(pacificVisit.count({point.first,point.second}) > 0 && atlanticVisit.count({point.first,point.second}) > 0){
                    res.push_back({point.first,point.second});
                }
            }
        }

        return res;
    }
};
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