多源BFS

#include<iostream>
#include<cstring>
#include<algorithm>
#include<queue>
#define x first
#define y second
using namespace std;
const int N = 1010;
typedef pair<int,int> PII;
int n,m;
int dist[N][N];
char g[N][N];
int dx[4] = {-1, 0, 1, 0}, dy[4] = {0, 1, 0, -1};

void bfs()
{
    memset(dist,-1,sizeof dist);
    queue<PII> q;
    for(int i = 0 ; i < n ; i++)
        for(int j = 0  ; j < m ; j++)
        {
            if(g[i][j]=='1')
            {
                dist[i][j] = 0 ;
                q.push({i,j});
            }
        }

    while(q.size())
    {
        PII t  = q.front();
        q.pop();
        for(int i = 0 ; i < 4 ; i++)
        {
            int a = t.x+dx[i];
            int b = t.y+dy[i];
            if(a < 0 || a >= n || b < 0 || b >= m || dist[a][b]!=-1) continue;
            dist[a][b] = dist[t.x][t.y]+1;
            q.push({a,b});
        }
        
    }
}
int main()
{
    cin >> n >> m;
    for(int i = 0 ; i < n ; i++) cin >> g[i];
    bfs();
    for(int i = 0 ; i < n ; i++)
    {
        for(int j = 0 ; j < m ; j++)
            printf("%d ",dist[i][j]);
        puts("");
    }
    return 0;
}