01 Matrix
<p>Given an <code>m x n</code> binary matrix <code>mat</code>, return <em>the distance of the nearest </em><code>0</code><em> for each cell</em>.</p>

<p>The distance between two cells sharing a common edge is <code>1</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0500-0599/0542.01%20Matrix/images/01-1-grid.jpg" style="width: 253px; height: 253px;" />
<pre>
<strong>Input:</strong> mat = [[0,0,0],[0,1,0],[0,0,0]]
<strong>Output:</strong> [[0,0,0],[0,1,0],[0,0,0]]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0500-0599/0542.01%20Matrix/images/01-2-grid.jpg" style="width: 253px; height: 253px;" />
<pre>
<strong>Input:</strong> mat = [[0,0,0],[0,1,0],[1,1,1]]
<strong>Output:</strong> [[0,0,0],[0,1,0],[1,2,1]]
</pre>

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

<ul>
	<li><code>m == mat.length</code></li>
	<li><code>n == mat[i].length</code></li>
	<li><code>1 &lt;= m, n &lt;= 10<sup>4</sup></code></li>
	<li><code>1 &lt;= m * n &lt;= 10<sup>4</sup></code></li>
	<li><code>mat[i][j]</code> is either <code>0</code> or <code>1</code>.</li>
	<li>There is at least one <code>0</code> in <code>mat</code>.</li>
</ul>

<p>&nbsp;</p>
<p><strong>Note:</strong> This question is the same as 1765: <a href="https://leetcode.com/problems/map-of-highest-peak/description/" target="_blank">https://leetcode.com/problems/map-of-highest-peak/</a></p>
Breadth-First Search; Array; Dynamic Programming; Matrix
func updateMatrix(mat [][]int) [][]int {
	m, n := len(mat), len(mat[0])
	ans := make([][]int, m)
	for i := range ans {
		ans[i] = make([]int, n)
		for j := range ans[i] {
			ans[i][j] = -1
		}
	}
	type pair struct{ x, y int }
	var q []pair
	for i, row := range mat {
		for j, v := range row {
			if v == 0 {
				ans[i][j] = 0
				q = append(q, pair{i, j})
			}
		}
	}
	dirs := []int{-1, 0, 1, 0, -1}
	for len(q) > 0 {
		p := q[0]
		q = q[1:]
		for i := 0; i < 4; i++ {
			x, y := p.x+dirs[i], p.y+dirs[i+1]
			if x >= 0 && x < m && y >= 0 && y < n && ans[x][y] == -1 {
				ans[x][y] = ans[p.x][p.y] + 1
				q = append(q, pair{x, y})
			}
		}
	}
	return ans
}