https://hydro.ac/d/bzoj/p/2252

#include <bits/stdc++.h>
using namespace std;
int n, m;
char mp[1005][1005];
int ans[1005][1005];
const int dx[] = {0, 0, 1, -1};
const int dy[] = {-1, 1, 0, 0};
struct Node {
	int x, y, step;
};
queue<Node> q;
void bfs() {
	int x; int y; int step;
	while (!q.empty()) {
		x = q.front().x;
		y = q.front().y;
		step = q.front().step;
		q.pop();
		for (int i = 0; i <= 3; i++) {
			int xx = x + dx[i], yy = y + dy[i];
			if (xx < 1 || yy < 1 || xx > n || yy > m) {
				continue;
			} 
			if (mp[xx][yy] == '1') {
				continue;
			}
			if (step + 1 >= ans[xx][yy]) {
				continue;
			}
			ans[xx][yy] = step + 1;
			q.push((Node){xx, yy, step + 1});
		}
	}
	return;
}
int main() {
	cin >> n >> m;
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= m; j++) {
			ans[i][j] = INT_MAX;
			cin >> mp[i][j];
		}
	}
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= m; j++) {
			if (mp[i][j] == '1') {
				ans[i][j] = 0;
				q.push((Node){i, j, 0});
				bfs();
			}
		}
	}
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= m; j++) {
			cout << ans[i][j] << " ";
		}
		cout << "\n";
	}
	return 0;
}
/*
3 4
0001
0011
0110
*/