<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:
bool isvalid(int x,int y,int n,int m){
    if((x>=0 && x<n) && (y>=0 && y<m))return true;
    return false;
}
    vector<vector<int>> updateMatrix(vector<vector<int>>& mat) {
        int n=mat.size(),m=mat[0].size();
        int INF=1000000;
        vector<vector<int>>vis(n,vector<int>(m,INF));
        queue<pair<int,int>>q;
        for(int i=0;i<n;i++){
            for(int j=0;j<m;j++){
                if(mat[i][j]==0){
             q.push({i,j});
             vis[i][j]=0;
                }
            }
        }
        while(!q.empty()){
            auto node=q.front();
            int a=node.first,b=node.second;
            q.pop();
            vector<pair<int,int>>p={{1,0},{-1,0},{0,1},{0,-1}};
            for(int i=0;i<4;i++){
                int x=a+p[i].first,y=b+p[i].second;
                if(isvalid(x,y,n,m)){
                    if(mat[a][b]==0 && mat[x][y]==1){
                        if((vis[a][b]+1)<vis[x][y]){
                            vis[x][y]=vis[a][b]+1;
                            q.push({x,y});
                        }
                    }
                    else if(mat[a][b]==1 && mat[x][y]==1){
                        if((vis[a][b]+1)<vis[x][y]){
                            vis[x][y]=vis[a][b]+1;
                            q.push({x,y});
                        }
                    }
                    else if(mat[a][b]==0 && mat[x][y]==0){
                        if(vis[a][b]<vis[x][y]){
                            vis[x][y]=vis[a][b];
                            q.push({x,y});
                        }
                    }
                    else{
                        vis[x][y]=0;
                        q.push({x,y});   
                    }
                }
            }
        }
        // for(int i=0;i<n;i++){
        //     for(int j=0;j<m;j++){
        //         cout<<vis[i][j]<<" ";
        //     }
        //     cout<<endl;
        // }
        return vis;
    }
};
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