hdu 4612——Warm up

tarjan縮點+樹的直徑

https://www.byvoid.com/blog/scc-tarjan/    講tarjian比較好

http://blog.csdn.net/u010638776/article/details/9472605   借鑒了這裡

 

#include<iostream>#include<cstdio>#include<cstdio>#include<queue>#pragma comment(linker, "/STACK:1024000000,1024000000")   //擴棧 using namespace std;#define maxn 210000#define maxm 2100000int n,m;int head[maxn],v[maxm],next[maxm],cnt;int head1[maxn],v1[maxm],next1[maxm],cnt1;int dfn[maxn],low[maxn],step;int sta[maxn],top;int ID[maxn],IdNum;int vis[maxn];int r;void Init() {memset(vis,0,sizeof(vis));memset(head1,-1,sizeof(head1));memset(head,-1,sizeof(head));memset(dfn,0,sizeof(dfn));memset(low,0,sizeof(low));IdNum=top=step=cnt1=cnt=0;}void add1(int a,int b){v1[cnt1]=b;next1[cnt1]=head1[a];head1[a]=cnt1++;}void add(int a,int b){v[cnt]=b;next[cnt]=head[a];head[a]=cnt++;}void tarjan(int u,int pre) //pre為前一個點 {dfn[u]=low[u]=++step;sta[++top]=u;vis[u]=1;int k=0;for(int i=head[u];~i;i=next[i]){int to=v[i];if(!dfn[to])//如果沒有進過棧 {tarjan(to,u);//繼續找 low[u]=min(low[u],low[to]);}else{if(to==pre)//如果是前一個點 {if(k)//考慮了重邊的可能 low[u]=min(low[u],dfn[to]);k++;//無向邊是建2條邊 }else if(vis[to])low[u]=min(low[u],dfn[to]);}}if(dfn[u]==low[u]){IdNum++;int x;do{x=sta[top--];vis[x]=0;ID[x]=IdNum;//表示在同一個強連通分量 }while(u!=x);}}int bfs(int x){queue<int> q;int dis[maxn];int end;memset(dis,-1,sizeof(dis));q.push(x);dis[x]=0;while(!q.empty()){int u=q.front();q.pop();r=dis[u];end=u;for(int i=head1[u];~i;i=next1[i]){int to=v1[i];if(dis[to]==-1){dis[to]=dis[u]+1;q.push(to);}}}return end;}int main(){int x,y;while(~scanf("%d%d",&n,&m)&&n+m){Init();for(int i=1;i<=m;i++){scanf("%d%d",&x,&y);if(x==y)continue;add(x,y);add(y,x);}tarjan(1,-1);//cout<<IdNum<<endl;for(int i=1;i<=n;i++)// 縮點建邊 for(int j=head[i];~j;j=next[j]){int to=v[j];if(ID[i]!=ID[to])add1(ID[i],ID[to]);}bfs(bfs(1));//2次bfs找樹的直徑 printf("%d\n",IdNum-1-r);}return 0;}