<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>

class Solution {
public:
    vector<vector<int>> updateMatrix(vector<vector<int>>& mat) {
        int m=mat.size();
        int n=mat[0].size();
        queue<vector<int>>q;
        vector<vector<bool>> vis(m,vector<bool>(n));
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(mat[i][j]==0){
                    q.push({i,j});
                    vis[i][j]=true;
                }
            }
        }
      
        int dir[5]={-1,0,1,0,-1};
        while(!q.empty()){
            int sz=q.size();
            while(sz--){           
                auto vec=q.front();
                int i=vec[0],j=vec[1];
                q.pop();
                for(int d=0;d<4;d++){
                    int x=i+dir[d];
                    int y=j+dir[d+1];
                    if(x<0 || x>=m || y<0 || y>=n ||
                    vis[x][y])continue;
                    mat[x][y]=mat[i][j]+1;
                    vis[x][y]=true;
                    q.push({x,y});
                }
             }
    
        }
        return mat;
    }
};
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