反著建邊，從T bfs找合法的點，然後再正著bfs一下求最短路就行了

 1 #include<bits/stdc++.h>
 2 #define pa pair<int,int>
 3 #define lowb(x) ((x)&(-(x)))
 4 #define REP(i,n0,n) for(i=n0;i<=n;i++)
 5 #define PER(i,n0,n) for(i=n;i>=n0;i--)
 6 #define MAX(a,b) ((a>b)?a:b)
 7 #define MIN(a,b) ((a<b)?a:b)
 8
 
 #define CLR(a,x) memset(a,x,sizeof(a))
 9 #define rei register int
10 using namespace std;
11 typedef long long ll;
12 const int maxn=10010,maxm=200020;
13 
14 inline ll rd(){
15     ll x=0;char c=getchar();int neg=1;
16     while(c<‘0‘||c>‘9‘){if(c==‘-‘) neg=-1;c=getchar();}
17     while(c>=‘
 
0‘&&c<=‘9‘) x=x*10+c-‘0‘,c=getchar();
18     return x*neg;
19 }
20 
21 int N,M,S,T;
22 int eg[maxm*2][2],egh[maxn],ect;
23 int neg[maxm*2][2],negh[maxn],nect,dis[maxn];
24 bool vis[maxn],used[maxn],flag[maxn];
25 queue<int> q;
26 
27 inline void adeg(int a,int b){
28     eg[++ect][0
 
]=b;eg[ect][1]=egh[a];egh[a]=ect;
29 }
30 inline void nadeg(int a,int b){
31     neg[++nect][0]=b;neg[nect][1]=negh[a];negh[a]=nect;
32 }
33 
34 void bfs1(){
35     q.push(T);
36     while(!q.empty()){
37         int p=q.front();q.pop();
38         if(vis[p]) continue;
39         vis[p]=1;
40         for(int i=negh[p];i;i=neg[i][1]){
41             int b=neg[i][0];
42             if(!vis[b]) q.push(b);
43         }
44     }
45     for(int i=1;i<=N;i++){
46         bool b=1;
47         for(int j=egh[i];j;j=eg[j][1]){
48             if(!vis[eg[j][0]]) b=0;
49         }used[i]=b;
50         // printf("!%d %d %d\n",i,used[i],vis[i]);
51     }
52 }
53 
54 int bfs2(){
55     if(used[S]) q.push(S);
56     dis[S]=0; flag[S]=1;
57     while(!q.empty()){
58         int p=q.front();q.pop();
59         if(p==T) return dis[p];
60         for(int i=egh[p];i;i=eg[i][1]){
61             if(!used[eg[i][0]]) continue;
62             if(!flag[eg[i][0]]) dis[eg[i][0]]=dis[p]+1,q.push(eg[i][0]),flag[eg[i][0]]=1;
63         }
64     }return -1;
65 }
66 
67 int main(){
68     rei i,j,k;
69     N=rd(),M=rd();
70     for(i=1;i<=M;i++){
71         int a=rd(),b=rd();
72         adeg(a,b);nadeg(b,a);
73     }S=rd(),T=rd();
74     bfs1();
75     printf("%d\n",bfs2());
76     
77     return 0;
78 }

luogu2296 [NOIp2014]尋找道路 (bfs)