class Solution {
    int m,n;
    int row[4]={1,-1,0,0};
    int col[4]={0,0,1,-1};
public:
    vector<vector<int>> updateMatrix(vector<vector<int>>& mat) {

        m=mat.size(),n=mat[0].size();
        vector<vector<int>> res(m,vector<int>(n,-1));
        vector<vector<bool>> visited(m,vector<bool>(n,false));

        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                visited[i][j]=true;
                disFromNearestZero(mat,i,j,res,visited);
                visited[i][j]=false;
            }
        }
        return res;
    }

    int disFromNearestZero(vector<vector<int>>& mat,int i,int j,vector<vector<int>> &res,vector<vector<bool>> &visited){

        //base case
        if(res[i][j] != -1)     return res[i][j];
        if(mat[i][j]==0)        {res[i][j]=0;return 0;}

        int minDis=INT_MAX;
        for(int k=0;k<4;k++){

            int x=i+row[k],y=j+col[k];
            if(isValid(x,y) && !visited[x][y]){
                visited[x][y]=true;
                minDis=min(minDis, 1+disFromNearestZero(mat,x,y,res,visited));
                visited[x][y]=false;
            }
        }
        res[i][j]=minDis;
        return minDis;
    }
    bool isValid(int i,int j){
        return i>=0 && j>=0 && i<m && j<n;
    }
};