NOIP 2014 寻找道路

思路：
反向建边 看能到哪儿

再正向搞
我傻乎乎地最后竟然用了Dijkstra 普通BFS就可以

//By SiriusRen
#include <queue>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
#define N 250000
struct node{int at,weight;friend bool operator < (node a,node b){return a.weight>b.weight;}}jy,tmp;
priority_queue<node>pq;
int n,first
,next
,v
,tot=0,start,end,dis
,rec
,m,xx
,yy
;
bool vis
;
void add(int x,int y){v[tot]=y,next[tot]=first[x],first[x]=tot++;}
void dfs(int x){
for(int i=first[x];~i;i=next[i]){
if(!vis[v[i]]){
vis[v[i]]=1;
dfs(v[i]);
}
}
}
bool check(int x){
for(int i=first[x];~i;i=next[i]){
if(!vis[v[i]])return 0;
}
return 1;
}
void Dijkstra(int x){
memset(dis,0x3f,sizeof(dis));
dis[x]=0;
jy.at=x;jy.weight=0;
pq.push(jy);
while(!pq.empty()){
jy=pq.top();pq.pop();
if(rec[jy.at])continue;
rec[jy.at]=1;
for(int i=first[jy.at];~i;i=next[i]){
if(!rec[v[i]]&&dis[v[i]]>dis[jy.at]+1&&vis[v[i]]&&check(v[i])){
dis[v[i]]=dis[jy.at]+1;
tmp.at=v[i];
tmp.weight=jy.weight+1;
pq.push(tmp);
}
}
}
}
int main(){
memset(first,-1,sizeof(first));
scanf("%d%d",&n,&m);
for(int i=1;i<=m;i++)
scanf("%d%d",&xx[i],&yy[i]),add(yy[i],xx[i]);
scanf("%d%d",&start,&end);
vis[end]=1;dfs(end);
memset(first,-1,sizeof(first));
tot=0;
for(int i=1;i<=m;i++)add(xx[i],yy[i]);
Dijkstra(start);
if(dis[end]<0x3ffffff)printf("%d",dis[end]);
else printf("-1");
}