AcWing 173 矩阵距离 （多源bfs）

#include <bits/stdc++.h>
using namespace std;

const int N = 1010, M = N * N;

int n, m;
char g[N][N];
int ans[N][N];
pair<int, int> q[M];

void bfs()
{
	int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
	int hh = 0, tt = -1;
	for (int i = 0; i < n; i ++ )
		for (int j = 0; j < m; j ++ )
			if (g[i][j] == '1')
			{
				ans[i][j] = 1;
				q[ ++ tt] = {i, j};
			}
	
	while (hh <= tt)
	{
		int tx = q[hh].first, ty = q[hh].second;
		hh ++;
		
		
		for (int i = 0; i < 4; i ++ )
		{
			int x = tx + dx[i], y = ty + dy[i];
			if (x < 0 || x >= n || y < 0 || y >= m) continue;
			if (!ans[x][y])
			{
				ans[x][y] = ans[tx][ty] + 1;
				q[++ tt ] = {x, y};
			}
		}
	}
	
	for (int i = 0; i < n; i ++ )
	{
		for (int j = 0; j < m; j ++ )
			printf("%d ", ans[i][j] - 1);
		puts("");
	}
}

int main()
{
	scanf("%d%d", &n, &m);
	
	for (int i = 0; i < n; i ++ )
	{
		scanf("%s", g[i]);
	}
	
	bfs();
	
	return 0;
}