寻找道路

  1 #include<cstdio>
  2 #include<algorithm>
  3 #include<cmath>
  4 #include<cstring>
  5 #include<iostream>
  6 #include<queue>
  7 using namespace std;
  8 //Debug Yufenglin
  9 #define dej printf("Running\n");
 10 #define dep1(x) cout<<#x<<"="<<x<<endl;
 11 #define dep2(x,y) cout<<#x<<"="<<x<<‘ ‘<<#y<<"="<<y<<endl;
 12 #define dep3(x,y,z) cout<<#x<<"="<<x<<‘ ‘<<#y<<"="<<y<<‘ ‘<<#z<<"="<<z<<endl;
 13 
 14 //Standfor Yufenglin
 15 #define LL long long
 16 #define LB long double
 17 #define reg register
 18 #define il inline
 19 #define maxn 10000
 20 #define maxm 200000
 21 #define inf 1000000000
 22 
 23 int n,m,cnt,s,t;
 24 struct EDGE
 25 {
 26     int to,nxt,v;
 27 };
 28 EDGE edge[maxm*2+5];
 29 bool canvis[maxn+5],vis[maxn+5],visited[maxn+5];
 30 int dis[maxn+5],head[maxn+5];
 31 queue<int>q;
 32 
 33 void add(int x,int y,int z)
 34 {
 35     edge[++cnt].to=y;
 36     edge[cnt].v=z;
 37     edge[cnt].nxt=head[x];
 38     head[x]=cnt;
 39 }
 40 void dfs(int x)
 41 {
 42     //canvis[x]=1;
 43     for(int i=head[x]; i; i=edge[i].nxt)
 44     {
 45         if(edge[i].v==-1&&canvis[edge[i].to]==0)
 46         {
 47             canvis[edge[i].to] = 1;
 48             dfs(edge[i].to);
 49         }
 50     }
 51 }
 52 void SPFA()
 53 {
 54     for(int i=1; i<=n; i++)
 55     {
 56         dis[i]=inf;
 57     }
 58     int u=s;
 59     dis[u]=0;
 60     vis[u]=1;
 61     q.push(u);
 62     while(!q.empty())
 63     {
 64         u=q.front();
 65         vis[u]=0;
 66         q.pop();
 67         for(int i=head[u]; i; i=edge[i].nxt)
 68         {
 69             if(visited[edge[i].to]==0) continue;
 70             if(dis[edge[i].to]>dis[u]+1)
 71             {
 72                 dis[edge[i].to]=dis[u]+1;
 73                 if(vis[edge[i].to]==0)
 74                 {
 75                     vis[edge[i].to]=1;
 76                     q.push(edge[i].to);
 77                 }
 78             }
 79         }
 80     }
 81 }
 82 int main()
 83 {
 84     scanf("%d%d",&n,&m);
 85     for(int i=1; i<=m; i++)
 86     {
 87         int x,y;
 88         scanf("%d%d",&x,&y);
 89         if(x==y) continue;
 90         add(x,y,1);
 91         add(y,x,-1);
 92     }
 93     scanf("%d%d",&s,&t);
 94     canvis[t] = 1;
 95     dfs(t);
 96     for(int i=1; i<=n; i++)
 97     {
 98         bool flag=1;
 99         for(int j=head[i];j;j=edge[j].nxt)
100         {
101             if(canvis[edge[j].to]==0)
102             {
103                 flag=0;
104                 break;
105             }
106         }
107         if(flag==1) visited[i]=1;
108     }
109     SPFA();
110     if(dis[t]==inf) printf("-1\n");
111     else printf("%d\n",dis[t]);
112     return 0;
113 }