luogu2296 [NOIp2014]寻找道路 (bfs)

反着建边，从T bfs找合法的点，然后再正着bfs一下求最短路就行了

 #include<bits/stdc++.h>
 #define pa pair<int,int>
 #define lowb(x) ((x)&(-(x)))
 #define REP(i,n0,n) for(i=n0;i<=n;i++)
 #define PER(i,n0,n) for(i=n;i>=n0;i--)
 #define MAX(a,b) ((a>b)?a:b)
 #define MIN(a,b) ((a<b)?a:b)
 #define CLR(a,x) memset(a,x,sizeof(a))
 #define rei register int
 using namespace std;
 typedef long long ll;
 const int maxn=,maxm=;

 inline ll rd(){
     ll x=;char c=getchar();int neg=;
     while(c<''||c>''){if(c=='-') neg=-;c=getchar();}
     while(c>=''&&c<='') x=x*+c-'',c=getchar();
     return x*neg;
 }

 int N,M,S,T;
 int eg[maxm*][],egh[maxn],ect;
 int neg[maxm*][],negh[maxn],nect,dis[maxn];
 bool vis[maxn],used[maxn],flag[maxn];
 queue<int> q;

 inline void adeg(int a,int b){
     eg[++ect][]=b;eg[ect][]=egh[a];egh[a]=ect;
 }
 inline void nadeg(int a,int b){
     neg[++nect][]=b;neg[nect][]=negh[a];negh[a]=nect;
 }

 void bfs1(){
     q.push(T);
     while(!q.empty()){
         int p=q.front();q.pop();
         if(vis[p]) continue;
         vis[p]=;
         for(int i=negh[p];i;i=neg[i][]){
             int b=neg[i][];
             if(!vis[b]) q.push(b);
         }
     }
     for(int i=;i<=N;i++){
         bool b=;
         for(int j=egh[i];j;j=eg[j][]){
             if(!vis[eg[j][]]) b=;
         }used[i]=b;
         // printf("!%d %d %d\n",i,used[i],vis[i]);
     }
 }

 int bfs2(){
     if(used[S]) q.push(S);
     dis[S]=; flag[S]=;
     while(!q.empty()){
         int p=q.front();q.pop();
         if(p==T) return dis[p];
         for(int i=egh[p];i;i=eg[i][]){
             if(!used[eg[i][]]) continue;
             if(!flag[eg[i][]]) dis[eg[i][]]=dis[p]+,q.push(eg[i][]),flag[eg[i][]]=;
         }
     }return -;
 }

 int main(){
     rei i,j,k;
     N=rd(),M=rd();
     for(i=;i<=M;i++){
         int a=rd(),b=rd();
         adeg(a,b);nadeg(b,a);
     }S=rd(),T=rd();
     bfs1();
     printf("%d\n",bfs2());

     return ;
 }