78 lines
1.6 KiB
C++
78 lines
1.6 KiB
C++
# include<stdio.h>
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# include<string.h>
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# define N 20005
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# define M 50005
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# include<stack>
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using namespace std;
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struct node{
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int from,to,next;
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}edge[M];
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int tol,head[N],dfn[N],low[N],flag,count,cnt,n;
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bool visit[N],vis[N];
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stack<int>S;
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void add(int a,int b)
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{
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edge[tol].from=a;edge[tol].to=b;edge[tol].next=head[a];head[a]=tol++;
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}
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int min(int a,int b)
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{
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return a<b?a:b;
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}
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void tarjan(int u)
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{
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int j,v;
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dfn[u]=low[u]=cnt++;
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visit[u]=vis[u]=1;
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S.push(u);
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for(j=head[u];j!=-1;j=edge[j].next)
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{
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v=edge[j].to;
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if(!visit[v])
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{
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tarjan(v);
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low[u]=min(low[u],low[v]);
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}
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else if(vis[v])
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{
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low[u]=min(low[u],dfn[v]);
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if(dfn[v]!=low[v]) flag=1;
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}
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if(flag) return;
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}
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if(dfn[u]==low[u])
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{
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count++;
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do{
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v=S.top();
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S.pop();
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vis[v]=0;
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}while(v!=u);
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}
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}
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int main()
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{
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int i,ncase,a,b;
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scanf("%d",&ncase);
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while(ncase--)
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{
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scanf("%d",&n);
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tol=0;
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memset(head,-1,sizeof(head));
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while(scanf("%d%d",&a,&b)!=EOF)
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{
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if(!a && !b) break;
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add(a,b);
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}
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memset(visit,0,sizeof(visit));
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memset(vis,0,sizeof(vis));
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flag=0;
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count=0;
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cnt=0;
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for(i=0;i<n;i++)
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if(!visit[i]) tarjan(i);
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if(flag||count!=1) printf("NO\n");
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else printf("YES\n");
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}
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return 0;
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}
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