算法提高课第二章多源BFS

有这样一个情景，对于很多源点，求得离他最近的目标的最短路径，称为多源最短路。这时可以创建一个虚拟源点，以边权为0连接所有的源点，这样就把多源最短路转化为了单源最短路

AcWing 173. 矩阵距离

#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>
using namespace std;
int n, m;
queue<pair<int, int>> q;
char a[1010][1010];
bool st[1010][1010];
int dis[1010][1010];
int bx[4] = {1,-1,0,0};
int by[4] = {0,0,1,-1};
void bfs()
{
    while(!q.empty())
    {
        auto t = q.front();
        int nowx = t.first, nowy = t.second;
        q.pop();
        for(int i = 0; i < 4; i ++ )
        {
            int nx = nowx + bx[i], ny = nowy + by[i];
            if(nx < 1 || ny < 1 || nx > n || ny > m )continue;
            if(st[nx][ny])continue;
            q.push({nx, ny});
            st[nx][ny] = true;
            dis[nx][ny] = dis[nowx][nowy] + 1;
        }
    }
}
int main()
{
    cin >> n >> m;
    for(int i = 1; i <= n; i ++ )
    for(int j = 1; j <= m; j ++ )
    {
        cin >> a[i][j];
        if(a[i][j] == '1')
        {
            q.push({i,j});
            st[i][j] = true;
        }
    }
    bfs();
    for(int i = 1; i <= n; i ++ )
    for(int j = 1; j <= m; j ++ )
    {
        printf("%d ", dis[i][j]);
        if(j == m)printf("\n");
    }
}