[J]computer network tarjan边双联通分量+树的直径

https://odzkskevi.qnssl.com/b660f16d70db1969261cd8b11235ec99?v=1537580031

【2012-2013 ACM Central Region of Russia Quarterfinal Programming Contest】【J】computer network

题意：

n个点，m条边，构成一个无向图，现在让你再任意连接两个点，使得整个图的割边最少。

1 ≤ n ≤ 10 000; 1≤ m ≤ 100 000; 1 ≤ xi, yi ≤ n; xi ≠ yi.

题解：
tarjan求边双联通分量，缩点，形成一棵树，求树的直径，然后把直径的两个端点连起来就好。

 #include<cstdio>
 #include<cstdlib>
 #include<cstring>
 #include<algorithm>
 #include<iostream>
 using namespace std;

 // const int maxN=100;
 // int a[maxN+10];
 const int N=,M=;
 int dfn[N],low[N],first[N],bfir[N],v[N],c[N],s[N],id[];
 int n,m,al,bl,sl,num,mx,cnt;
 struct node{
     int x,y,next,tmp;
 }a[*M],b[*M];

 int minn(int x,int y){return x<y ? x:y;}

 void ins(int x,int y)
 {
     al++;a[al].tmp=;
     a[al].x=x;a[al].y=y;a[al].next=first[x];first[x]=al;
 }

 void anins(int x,int y)
 {
     bl++;b[bl].tmp=;
     b[bl].x=x;b[bl].y=y;b[bl].next=bfir[x];bfir[x]=bl;
 }

 void tarjan(int x,int fa)
 {
     dfn[x]=low[x]=++num;
     s[++sl]=x;
     for(int i=first[x];i;i=a[i].next)
     {
         if(a[i].tmp) continue;
         a[i%== ? i-:i+].tmp=;
         int y=a[i].y;
         if(!dfn[y])
         {
             tarjan(y,x);
             low[x]=minn(low[x],low[y]);
         }
         else if(!c[y]) low[x]=minn(low[x],dfn[y]);
     }
     if(dfn[x]==low[x])
     {
         cnt++;
         int z=;
         while()
         {
             z=s[sl];sl--;
             v[z]=;
             c[z]=cnt;
             if(z==x) break;
         }
     }
 }

 void dfs(int x,int fa,int dep,int tmp)
 {
     if(dep>mx) mx=dep,id[tmp]=x;
     for(int i=bfir[x];i;i=b[i].next)
     {
         int y=b[i].y;
         if(y==fa) continue;
         dfs(y,x,dep+,tmp);
     }
 }

 int main()
 {
     freopen("input.txt","r",stdin);
     freopen("output.txt","w",stdout);
     scanf("%d%d",&n,&m);
     al=;sl=;bl=;num=;cnt=;
     memset(bfir,,sizeof(bfir));
     memset(first,,sizeof(first));
     memset(dfn,,sizeof(dfn));
     memset(low,,sizeof(low));
     memset(c,,sizeof(c));
     for(int i=;i<=m;i++)
     {
         int x,y;
         scanf("%d%d",&x,&y);
         ins(x,y);ins(y,x);
     }
     for(int i=;i<=n;i++)
     {
         if(!dfn[i]) num=,tarjan(i,);
     }
     // for(int i=1;i<=n;i++) printf("c [ %d ] = %d\n",i,c[i]);
     for(int i=;i<=*m;i++)
     {
         int x=a[i].x,y=a[i].y;
         if(c[y]==c[x]) continue;
         anins(c[x],c[y]);
         // printf("%d -- > %d\n",c[x],c[y]);
     }
     mx=,dfs(,,,);
     mx=,dfs(id[],,,);
     for(int i=;i<=n;i++)
         if(c[i]==id[]) {printf("%d ",i);break;}
     for(int i=;i<=n;i++)
         if(c[i]==id[]) {printf("%d\n",i);break;}
     // printf("%d %d\n",id[0],id[1]);
     return ;
 }