hdu4612（双连通缩点+树的直径）

#pragma comment(linker,"/STACK:1024000000,1024000000")
#include <cstdio>
#include <cstring>
#include <string>
#include <cmath>
#include <iostream>
#include <algorithm>
#include <queue>
#include <cstdlib>
#include <stack>
#include <vector>
#include <set>
#include <map>
#define LL long long
#define mod 100000000
#define inf 0x3f3f3f3f
#define eps 1e-6
#define N 200010
#define FILL(a,b) (memset(a,b,sizeof(a)))
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define PII pair<int,int>
using namespace std;
struct edge
{
    int v,next;
    edge(){}
    edge(int v,int next):v(v),next(next){}
}e[N*10],e2[N*10];
int n,step,scc,top,tot;
int head[N],dfn[N],low[N],belong[N],Stack[N];
bool instack[N],vis[N*10];
vector<int>g[N];
void init()
{
    tot=0;top=0;scc=0;
    FILL(head,-1);FILL(dfn,0);
    FILL(low,0);FILL(instack,false);
    FILL(vis,false);
}
void addedge(int u,int v)
{
    e[tot]=edge(v,head[u]);
    head[u]=tot++;
}
void tarjan(int u)
{
    int v;
    dfn[u]=low[u]=++step;
    Stack[top++]=u;
    instack[u]=true;
    int pre_num=0;
    for(int i=head[u];~i;i=e[i].next)
    {
        v=e[i].v;
        if(vis[i])continue;
        vis[i]=vis[i^1]=true;
        if(!dfn[v])
        {
            tarjan(v);
            if(low[u]>low[v])low[u]=low[v];
        }
        else if(instack[v])
        {
            if(low[u]>dfn[v])low[u]=dfn[v];
        }
    }
    if(dfn[u]==low[u])
    {
        scc++;
        do
        {
            v=Stack[--top];
            instack[v]=false;
            belong[v]=scc;
        }while(v!=u);
    }
}
int tree_len,x;
void dfs_tree(int u,int f,int d)
{
    if(d>=tree_len)tree_len=d,x=u;
    for(int i=0,sz=g[u].size();i<sz;i++)
    {
        int v=g[u][i];
        if(v==f)continue;
        dfs_tree(v,u,d+1);
    }
}
void solve()
{
    for(int i=1;i<=n;i++)
        if(!dfn[i])tarjan(i);
    for(int i=1;i<=n;i++)g[i].clear();
    for(int u=1;u<=n;u++)
    {
        for(int i=head[u];~i;i=e[i].next)
        {
            int v=e[i].v;
            if(belong[u]!=belong[v])
            {
                g[belong[u]].push_back(belong[v]);
            }
        }
    }
    tree_len=0;
    dfs_tree(1,-1,0);
    dfs_tree(x,-1,0);
    printf("%d\n",scc-1-tree_len);
}
int main()
{
    int m,u,v;
    while(scanf("%d%d",&n,&m)>0)
    {
        if(n+m==0)break;
        init();
        for(int i=1;i<=m;i++)
        {
            scanf("%d%d",&u,&v);
            addedge(u,v);
            addedge(v,u);
        }
        solve();
    }
}