hdu 4612 Warm up 双连通缩点+树的直径

首先双连通缩点建立新图（顺带求原图的总的桥数，事实上因为原图是一个强连通图，所以桥就等于缩点后的边）

此时得到的图类似树结构，对于新图求一次直径，也就是最长链。

我们新建的边就一定是连接这条最长链的首尾，这样就将原图的桥降低了直径个。

#include<iostream>
#include<cstring>
#include<cstdio>
#include<vector>
#include<algorithm>
#include<map>
using namespace std;
#pragma comment(linker, "/STACK:102400000,102400000")
#define maxn 200005
#define maxm 2000005
struct node
{
    int to,vis,next;
}e[maxm],e2[maxm];
int head[maxn],head2[maxn],en,en2;
int belong[maxn],vis[maxn],dfn[maxn],low[maxn],cnt,bridge,col,stack[maxn],top;
bool use[maxn];
void add(int a,int b)
{
    e[en].to=b;
    e[en].vis=0;
    e[en].next=head[a];
    head[a]=en++;
    e[en].to=a;
    e[en].vis=0;
    e[en].next=head[b];
    head[b]=en++;
}
void add2(int a,int b)
{
    e2[en2].to=b;
    e2[en2].next=head2[a];
    head2[a]=en2++;
    e2[en2].to=a;
    e2[en2].next=head2[b];
    head2[b]=en2++;
}
void init()
{
    top=cnt=col=bridge=en2=en=0;
    memset(vis,0,sizeof(vis));
    memset(dfn,0,sizeof(dfn));
    memset(low,0,sizeof(low));
    memset(use,0,sizeof(use));
    memset(head2,-1,sizeof(head2));
    memset(head,-1,sizeof(head));
}
void Tarjan (int u)
{
    int v;
    vis[u] = 1;
    dfn[u] = low[u] = ++cnt;
    stack[top++] = u;
    for (int i = head[u];i != -1;i = e[i].next)
    {
        v = e[i].to;
        if (e[i].vis) continue;
        e[i].vis = e[i^1].vis = 1;
        if (vis[v] == 1)
            low[u] = min(low[u],dfn[v]);
        if (!vis[v])
        {
            Tarjan (v);
            low[u] = min(low[u],low[v]);
            if (low[v] > dfn[u])
                bridge ++;
        }
    }
    if (dfn[u] == low[u])
    {
        ++col;
        do{
            v = stack[--top];
            vis[v] = 0;
            belong[v] = col;
        }while (u != v);
    }
}
int len,st;
void dfs(int now,int sum,int fa)
{
    use[now]=1;
    if(sum>len) {st=now;len=sum;}
    for(int i=head2[now];~i;i=e2[i].next)
    {
        if(!use[e2[i].to]) dfs(e2[i].to,sum+1,now);
    }
}
inline int ReadInt()
{
    char ch = getchar();
    int data = 0;
    while (ch < '0' || ch > '9')
    {
        ch = getchar();
    }
    do
    {
        data = data*10 + ch-'0';
        ch = getchar();
    }while (ch >= '0' && ch <= '9');
        return data;
}
int main()
{
    int n,m,a,b;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        if(!n&&!m) break;
        init();
        for(int i=1;i<=m;i++)
        {
            a=ReadInt();
            b=ReadInt();
            add(a,b);
        }
        Tarjan(1);
        for(int i=1;i<=n;i++)
            for(int j=head[i];~j;j=e[j].next)
            {
                int to=e[j].to;
                if(belong[to]!=belong[i]) {add2(belong[to],belong[i]);}
            }
        len=0;dfs(1,0,-1);
        memset(use,0,sizeof(use));
        len=0;dfs(st,0,-1);
        printf("%d
",bridge-len);
    }
    return 0;
}