<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 adjacent cells is <code>1</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/04/24/01-1-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://assets.leetcode.com/uploads/2021/04/24/01-2-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>

from queue import Queue
class Solution:
    def updateMatrix(self, mat: List[List[int]]) -> List[List[int]]:
        rows=len(mat)
        cols=len(mat[0])
        visited=[[False for _ in range(cols)] for _ in range(rows)]
        ans=[[0 for _ in range(cols)] for _ in range(rows)]
        
        q=Queue()
        for i in range(rows):
            for j in range(cols):
                if not visited[i][j] and mat[i][j]==0:
                    q.put((i,j,0))
                    visited[i][j]=True
        
        while not q.empty():
            row,col,dist=q.get()
            
            ans[row][col]=dist
            for dr,dc in [(0,1),(0,-1),(1,0),(-1,0)]:
                newrow=row+dr
                newcol=col+dc
                if 0<=newrow<len(mat) and 0<=newcol<len(mat[0]) and not visited[newrow][newcol] and mat[newrow][newcol]==1:
                    q.put((newrow,newcol,dist+1))
                    visited[newrow][newcol]=True

            
        return ans
20800