题解 | #【模板】单源最短路1#

#include <iostream>
#include <algorithm>
#include <cstring>
using namespace std;
const int N = 5e4+10;
int h[N],d[N],q[N];
int e[2*N],ne[2*N],idx;
int n,m;

void add(int a,int b)
{
    e[idx] = b,ne[idx] = h[a],h[a] = idx++;
}

int bfs()
{
    memset(d,-1,sizeof d);
    int hh = 0,tt = 0;
    d[1] = 0;
    q[0] = 1;

    while(hh<=tt)
    {
        int t = q[hh++];
        for(int i = h[t];i != -1;i = ne[i])
        {
            int j = e[i];
            if(d[j] == -1)
            {
                d[j] = d[t] + 1;
                q[++tt] = j; 
            }
        }
    }
    return d[n];
}

int main()
{
    cin>>n>>m;
    memset(h,-1,sizeof h);
    for(int i = 0;i<m;i++)
    {
        int u,v;
        cin>>u>>v;
        add(u,v);add(v,u);
    }
    cout << bfs() << endl;
    return 0;
}