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#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <cmath>
#include <unordered_map>
#include <unordered_set>
#include <string>
#include <sstream>
#include <climits>
#define x first
#define y second
#define pub push_back
#define mp make_pair
#define ll long long
using namespace std;
typedef pair<int, int> PII;
struct node{
int x, y;
node(){}
node(int _x = 0,int _y = 0): x(_x), y(_y){}
};
int dir[4][2] = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
int n;
vector<string> graph;
vector<vector<int>> vis;
int solve() {
queue<node> Q;
for(int i = 0;i < n;++i){
for(int j = 0;j < n;++j){
if(graph[i][j] == '#') Q.push(node(i,j));
vis[i][j] = 0;
}
}
if(Q.size() == n * n || Q.empty()) return -1;
int dis = 0;
while(!Q.empty()){
int size = Q.size();
while(size-- > 0){
node cur = Q.front();
Q.pop();
for(int i = 0;i < 4;i++){
int nextX = cur.x + dir[i][0];
int nextY = cur.y + dir[i][1];
if(nextX > -1 && nextX < n && nextY > -1 && nextY < n && graph[nextX][nextY] == '.' && !vis[nextX][nextY]){
vis[nextX][nextY] = 1;
Q.push(node(nextX,nextY));
}
}
}
if(!Q.empty()) dis += 1;
}
return dis;
}
int main(void) {
cin >> n;
string line;
vis.resize(n + 1, vector<int>(n + 1));
for (int i = 0; i < n; i++) {
cin >> line;
graph.push_back(line);
}
cout << solve();
return 0;
}
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