寻找道路 vijos1909 NOIP2014 D2T2 SPFA

  1 #include<cstdio>
  2 #include<cstring>
  3 #include<algorithm>
  4 #include<cmath>
  5 #include<queue>
  6 #include<iostream>
  7 using namespace std;
  8 template<class T> inline void read(T &_a){
  9     bool f=0;int _ch=getchar();_a=0;
 10     while(_ch<‘0‘ || _ch>‘9‘){if(_ch==‘-‘)f=1;_ch=getchar();}
 11     while(_ch>=‘0‘ && _ch<=‘9‘){_a=(_a<<1)+(_a<<3)+_ch-‘0‘;_ch=getchar();}
 12     if(f)_a=-_a;
 13 }
 14 
 15 const int maxn=10001,maxm=200001;
 16 struct edge
 17 {
 18     int to,next;
 19 }w[maxm],w2[maxm];
 20 int n,m,egcnt,egcnt2,head[maxn],head2[maxn],s,t,dis2[maxn],dis[maxn];
 21 queue<int>q;
 22 bool ins[maxn],able[maxn];
 23 
 24 inline void addedge(int from,int to)
 25 {
 26     w[++egcnt].to=to;
 27     w[egcnt].next=head[from];
 28     head[from]=egcnt;
 29     w2[++egcnt2].to=from;
 30     w2[egcnt2].next=head2[to];
 31     head2[to]=egcnt2;
 32 }
 33 
 34 inline void spfa2()
 35 {
 36     q.push(t);
 37     memset(dis2,0x7f,sizeof(dis2));
 38     dis2[t]=0;
 39     while(!q.empty())
 40     {
 41         int now=q.front(); q.pop();
 42         ins[now]=false;
 43         for (register int i=head2[now];i;i=w2[i].next)
 44         {
 45             if(dis2[w2[i].to]>dis2[now]+1)
 46             {
 47                 dis2[w2[i].to]=dis2[now]+1;
 48                 if(!ins[w2[i].to])
 49                 {
 50                     q.push(w2[i].to);
 51                     ins[w2[i].to]=true;
 52                 }
 53             }
 54         }
 55     }
 56 }
 57 
 58 inline bool check(int u)
 59 {
 60     if(!able[u]) return false;
 61     for (register int i=head[u];i;i=w[i].next)
 62         if(!able[w[i].to]) return false;
 63     return true;
 64 }
 65 
 66 inline void spfa()
 67 {
 68     while(!q.empty()) q.pop();
 69     memset(ins,0,sizeof(ins));
 70     q.push(s);
 71     memset(dis,0x7f,sizeof(dis));
 72     dis[s]=0;
 73     while(!q.empty())
 74     {
 75         int now=q.front(); q.pop();
 76         ins[now]=false;
 77         for (register int i=head[now];i;i=w[i].next)
 78             if(check(w[i].to)){
 79             if(dis[w[i].to]>dis[now]+1)
 80             {
 81                 dis[w[i].to]=dis[now]+1;
 82                 if(!ins[w[i].to])
 83                 {
 84                     q.push(w[i].to);
 85                     ins[w[i].to]=true;
 86                 }
 87             }
 88         }
 89     }
 90 }
 91 
 92 int main()
 93 {
 94     read(n); read(m);
 95     for (register int i=1,x,y;i<=m;++i) read(x),read(y),addedge(x,y);
 96     read(s); read(t);
 97     spfa2();
 98     for (register int i=1;i<=n;++i) if(dis2[i]<=n) able[i]=true;
 99     spfa();
100     printf("%d",dis[t]>n?-1:dis[t]);
101     return 0;
102 }