题解 | #【模板】单源最短路1#

#include <iostream>
#include <cstring>
#include <queue>
using namespace std;

typedef long long LL;

const int N = 5050;

struct edge{int v,w;};
vector <edge> e[N];

priority_queue <pair<LL,LL>> q;

bool vis[N];
LL d[N];
int n,m;


void dijkstra(int s){
  memset(d,-1,sizeof d);
  d[s]=0;
  q.push({0,s});
  while(q.size()){
    auto t = q.top();
    q.pop();

    int u =t.second;
    if(vis[u]) continue;
    vis[u]= true;

    for(auto ed:e[u]){
      LL v=ed.v,w=ed.w;
      if(d[v]>d[u]+w||d[v]==-1){
        d[v] =d[u]+w;
        q.push({-d[v],v});
      }
    }

  }

}

int main(){
    cin>>n>>m;
    while(m--){
        int a,b;
        if(a>n||b>n) continue;
        cin>>a>>b;
        e[a].push_back({b,1});
        e[b].push_back({a,1});
    }

    dijkstra(n);
//无向图
    cout<<d[1]<<endl;

    return 0;
}