88 lines
1.7 KiB
C++
88 lines
1.7 KiB
C++
#include<cstdio>
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#include<cstring>
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#include<queue>
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#include<algorithm>
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using namespace std;
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const int maxn=10005;
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const int maxm=200005;
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const int oo=0x3f3f3f3f;
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struct Graph
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{
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struct Edge { int v; Edge *next; };
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Edge mem[maxm],*G[maxn],*ecnt;
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Graph() { ecnt=mem; }
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inline void AddEdge(int u,int v) { ecnt->v=v; ecnt->next=G[u]; G[u]=ecnt++; }
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};
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Graph G,inv;
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int n,m,x,y,s,t;
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int d[maxn]; bool can[maxn],valid[maxn],vis[maxn];
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void init()
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{
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valid[t]=true;
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queue<int> Q;
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Q.push(t); vis[t]=true;
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while(Q.size())
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{
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int u=Q.front(); Q.pop();
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for(Graph::Edge *it=inv.G[u];it;it=it->next)
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if(!vis[it->v])
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{
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Q.push(it->v);
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vis[it->v]=true; valid[it->v]=true;
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}
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}
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for(int i=1;i<=n;i++)
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{
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can[i]=(G.G[i]!=0);
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if(!can[i]) continue;
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for(Graph::Edge *it=G.G[i];it;it=it->next)
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if(!valid[it->v])
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{
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can[i]=false;
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break;
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}
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}
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can[t]=true;
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}
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struct Node { int u,dis; };
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inline bool operator<(const Node &a,const Node &b) { return a.dis>b.dis; }
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int dijskra()
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{
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memset(vis,0,sizeof(vis));
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memset(d,0x3f,sizeof(d));
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if(!can[s]) return -1;
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d[s]=0;
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priority_queue<Node> Q;
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Q.push(Node{s,0});
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while(Q.size())
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{
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int u=Q.top().u,dis=Q.top().dis; Q.pop();
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if(vis[u]) continue;
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for(Graph::Edge *it=G.G[u];it;it=it->next)
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if(can[it->v]&&dis+1<d[it->v])
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{
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d[it->v]=dis+1;
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Q.push(Node{it->v,d[it->v]});
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}
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}
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return d[t]<oo?d[t]:-1;
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}
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int main()
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{
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#ifdef local
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freopen("pro.in","r",stdin);
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#endif
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scanf("%d%d",&n,&m);
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while(m-->0)
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{
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scanf("%d%d",&x,&y);
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G.AddEdge(x,y); inv.AddEdge(y,x);
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}
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scanf("%d%d",&s,&t);
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init();
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// for(int i=1;i<=n;i++) printf("valid[%d]=%d ",i,valid[i]); printf("\n");
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// for(int i=1;i<=n;i++) printf("can[%d]=%d ",i,can[i]); printf("\n");
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printf("%d\n",dijskra());
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return 0;
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}
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