题解

#include <bits/stdc++.h>
using namespace std;
 
const int Maxn=10005;
const int Maxm=200005;
const int inf=1e9;
int n,m,size,s,t;
int first[Maxn],First[Maxn],v[Maxn],pre[Maxn],dis[Maxn],to[Maxn],vis[Maxn];
struct shu{int to,next;};
shu edge[Maxm<<1],Edge[Maxm<<1];
 
inline int get_int()
{
	int x=0,f=1;
	char c;
	for(c=getchar();(!isdigit(c))&&(c!='-');c=getchar());
	if(c=='-') f=-1,c=getchar();
	for(;isdigit(c);c=getchar()) x=(x<<3)+(x<<1)+c-'0';
	return x*f;
}
 
inline void build(int x,int y)
{
	edge[++size].next=first[x];
	first[x]=size;
	edge[size].to=y;
	Edge[size].next=First[y];
	First[y]=size;
	Edge[size].to=x;
}
 
inline void dfs(int point)
{
	for(int u=First[point];u;u=Edge[u].next)
	{
	  int to=Edge[u].to;
	  first[to]=0;
	}
}
 
inline void dijkstra()
{
	priority_queue<pair<int,int> >q;
	for(int i=1;i<=n;i++) dis[i]=inf;
	dis[s]=0,q.push(make_pair(0,s));
	while(q.size())
	{
	  int point=q.top().second;
	  q.pop();if(vis[point]) continue;vis[point]=1;
	  for(int u=first[point];u;u=edge[u].next)
	  {
	  	int to=edge[u].to;
	  	if(dis[to] > dis[point] + 1)
	  	{
	  	  dis[to]=dis[point]+1;
	  	  q.push(make_pair(-dis[to],to));
	  	}
	  }
	}
}
 
int main()
{
	n=get_int(),m=get_int();
	for(int i=1;i<=m;i++)
	{
	  int x=get_int(),y=get_int();
	  build(x,y),to[x]++;
	}
	s=get_int(),t=get_int();
	for(int i=1;i<=n;i++) if(!to[i] && i!=t) dfs(i);
	dijkstra();
	if(dis[t]==inf) cout<<"-1\n";
	else cout<<dis[t]<<"\n";
	return 0;
}