[NOIP2014]寻找道路 题解

 1 #include<cstdio>
 2 const int M=200005,INF=10000000;
 3 int cnt,n,m,s,t,i,j,o[M],q[M],v[M],w[M],dist[M],last[M],head[M];
 4 bool c[M],g[M],vis[M];
 5 
 6 void add(int a,int b) { v[++cnt]=b,last[cnt]=head[a],head[a]=cnt,w[cnt]=1; }
 7 
 8 void bfs(int s)
 9 {
10     int h=0,t;
11     for (q[t=1]=s,g[s]=1;h<t;)
12     {
13         if (++h>=M) h=h-M;
14         int u=q[h],i;
15         for (i=head[u];i;i=last[i])
16             if (!g[v[i]])
17             {
18                 g[v[i]]=1;
19                 if (++t>=M) t=t-M;
20                 q[t]=v[i];
21             }
22     }
23 }
24 
25 int SPFA(int s,int t)
26 {
27     int l=0,r=1,i;
28     for (i=1;i<=n;i++) dist[i]=INF;
29     for (vis[s]=1,q[1]=s,dist[s]=0;l<r;)
30     {
31         if (++l>=M) l=l-M;
32         int u=q[l],i,x; vis[u]=0;
33         if (++o[u]>n) return -1;
34         for (i=head[u];i;i=last[i])
35             if (!c[v[i]] && dist[x=v[i]]>dist[u]+w[i])
36             {
37                 dist[x]=dist[u]+w[i];
38                 if (!vis[x])
39                 {
40                     vis[x]=1;
41                     if (++r>=M) r=r-M;
42                     q[r]=x;
43                 }
44             }
45     }
46     return dist[t]<INF?dist[t]:-1;
47 }
48 
49 int main()
50 {
51     scanf("%d%d",&n,&m);
52     for (i=1;i<=m;++i) scanf("%d%d",&s,&t),add(t,s);
53     scanf("%d%d",&s,&t);
54     bfs(t);
55     for (i=1;i<=n;i++)
56         if (!g[i])
57             for (c[i]=1,j=head[i];j;j=last[j]) c[v[j]]=1;
58     printf("%d\n",SPFA(t,s));
59     return 0;
60 }