题解 | 【模板】单源最短路Ⅰ ‖ 无权图：BFS

#include<bits/stdc++.h>
using namespace std;
using ll=long long;
const ll N=2E5+5;
vector<vector<ll>>adj(N);
queue<ll>q;
vector<ll>vis(N,-1);

void bfs(){

    while(!q.empty()){
        ll t=q.front();
        q.pop();

        for(auto it:adj[t]){
            if(vis[it]==-1){
                vis[it]=vis[t]+1;
                q.push(it);
            }
        }
    }
}


int main(){
	ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
    ll n,m,s;
    cin>>n>>m>>s;
    for(ll i=1;i<=m;i++){
        ll u,v;
        cin>>u>>v;
        adj[u].emplace_back(v);
    }
    q.push(s);
    vis[s]=0;
    bfs();
    for(ll i=1;i<=n;i++){
        cout<<vis[i]<<' ';
    }

	return 0;
}