【洛谷P2296】寻找道路

反正图两边bfs

 #include<iostream>
 #include<cstdio>
 #include<queue>

 using namespace std;

 inline int readint(){
     int x=,f=;
     char ch=getchar();
     while (ch<''||ch>''){
         if (ch=='-') f=-;
         ch=getchar();
     }
     while (ch>=''&&ch<=''){
         x=(x<<)+(x<<)+ch-'';
         ch=getchar();
     }
     return x*f;
 }
 struct edge{
     int x,nxt;
 }e1[],e2[];

 int n,m,s,t,t1,t2,kk,inq[],v[],tmp,sg=,v2[],v1[],cnt,head1[],head2[];
 struct node{
     int x,p;
 }now;
 inline void adde1(int a,int b){
     ++cnt;
     e1[cnt].x=b;
     e1[cnt].nxt=head1[a];
     head1[a]=cnt;
 }inline void adde2(int a,int b){
     ++cnt;
     e2[cnt].x=b;
     e2[cnt].nxt=head2[a];
     head2[a]=cnt;
 }
 queue <node> q;
 queue <int> qq;
 int main(){
     n=readint();
     m=readint();
     for (int i=;i<=m;++i){
         t1=readint();
         t2=readint();
         adde1(t1,t2);
         adde2(t2,t1);
     }
     s=readint();
     t=readint();
     kk=t;
     inq[kk]=;
     qq.push(kk);
     while (!qq.empty()){
         kk=qq.front();
         qq.pop();
         inq[kk]=;
         v1[kk]=;
         for (int i=head2[kk];i;i=e2[i].nxt){
             tmp=e2[i].x;
             if ((!inq[tmp])&&(!v1[tmp])) qq.push(tmp);
         }
     }
     for (int i=;i<=n;++i) v[i]=v1[i];
     for (int i=;i<=n;++i) {
         if (v1[i]==) {
             v[i]=;
             for (int j=head2[i];j;j=e2[j].nxt){
                 v[e2[j].x]=;
             }
         }

     }
     q.push((node){s,});
     while (!q.empty()){
         now=q.front();
         q.pop();
         v2[now.x]=;
         if (now.x==t) printf("%d",now.p),sg=;
         if (sg==) break;
         for (int i=head1[now.x];i;i=e1[i].nxt){
             tmp=e1[i].x;
             if (v[tmp]==&&v2[tmp]==) {
                 q.push((node){tmp,now.p+});
             }
         }
     }
     if (sg==) printf("-1\n");
 }

STD