经典bfs

#include<bits/stdc++.h> 
using namespace std;
int n,m;
vector<int>G[210000];
queue<int>Q;
int b[210000];
int main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++){
		int x,y;
		scanf("%d%d",&x,&y);
		G[x].emplace_back(y);
	}
	Q.emplace(1);
	b[1]=0;
	while(Q.size()){
		int x=Q.front();
		Q.pop();
		for(auto y:G[x])if(!b[y]){
			Q.emplace(y);
			b[y]=b[x]+1;
		}
	}
	if(b[1]==0)printf("-1");
	else printf("%d",b[1]);
}

跑dijkstra最短路即可

#include<bits/stdc++.h>
using namespace std;
const int N=2e5+10;
int d[N],v[N];vector<int>G[N];
#define PII pair<int,int>
#define fi first
#define se second
void dij()
{
	memset(d,0x3f,sizeof(d));
	priority_queue<PII,vector<PII>,greater<PII> >q;
	q.push({0,1});v[1]=1;d[1]=0;
	while(!q.empty())
	{
		int x=q.top().se;q.pop();
		for(int y:G[x])
		{
			if(d[y]>d[x]+1)
			{
				d[y]=d[x]+1;
				if(!v[y]){v[y]=1;q.push({d[y],y});}
			}
			if(y==1){cout<<d[x]+1;exit(0);}
		}
	}
}
int main()
{
	int n,m;cin>>n>>m;
	for(int i=1;i<=m;i++)
	{
		int x,y;cin>>x>>y;
		G[x].push_back(y);
	}
	dij();cout<<-1;
	return 0; 
}