Pacific Atlantic Water Flow
<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://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0400-0499/0417.Pacific%20Atlantic%20Water%20Flow/images/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>
Depth-First Search; Breadth-First Search; Array; Matrix
func pacificAtlantic(heights [][]int) [][]int {
	m, n := len(heights), len(heights[0])
	vis1 := make(map[int]bool)
	vis2 := make(map[int]bool)
	var q1 [][]int
	var q2 [][]int
	for i := 0; i < m; i++ {
		for j := 0; j < n; j++ {
			if i == 0 || j == 0 {
				vis1[i*n+j] = true
				q1 = append(q1, []int{i, j})
			}
			if i == m-1 || j == n-1 {
				vis2[i*n+j] = true
				q2 = append(q2, []int{i, j})
			}
		}
	}
	dirs := []int{-1, 0, 1, 0, -1}
	bfs := func(q [][]int, vis map[int]bool) {
		for len(q) > 0 {
			for k := len(q); k > 0; k-- {
				p := q[0]
				q = q[1:]
				for i := 0; i < 4; i++ {
					x, y := p[0]+dirs[i], p[1]+dirs[i+1]
					if x >= 0 && x < m && y >= 0 && y < n && !vis[x*n+y] && heights[x][y] >= heights[p[0]][p[1]] {
						vis[x*n+y] = true
						q = append(q, []int{x, y})
					}
				}
			}
		}
	}
	bfs(q1, vis1)
	bfs(q2, vis2)
	var ans [][]int
	for i := 0; i < m; i++ {
		for j := 0; j < n; j++ {
			x := i*n + j
			if vis1[x] && vis2[x] {
				ans = append(ans, []int{i, j})
			}
		}
	}
	return ans
}