Hdu 4612 Warm up (双连通缩点+树的直径)

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include <cstdio>
#include <cstring>
#include <stack>
#include <queue>
#include <vector>
#define min(x,y) ((x)<(y)?(x):(y))
#define max(x,y) ((x)>(y)?(x):(y))
using namespace std;

const int INF=0x3f3f3f3f;

const int nPoint=200010;
const int nEdges=2000010;
vector<int>G[nPoint];

class BCC
{
public:
	struct Edge{
		int from, to, next;
		bool cut;  //是否为桥
	}edge[nEdges];

	int e,id,n;
	int head[nPoint],dfn[nPoint], low[nPoint];
	int bridgetop;
	int colornum, top; //双连通分量数,栈顶
	int color[nPoint],Stack[nPoint];
	bool iscut[nPoint];  //该点是否为割点
	int bri_cut;    //桥的数目

	void Add (int u, int v){
		Edge E={u,v,head[u],false};
		edge[ e ] = E;
		head[u] = e++;
	}

	void Tarjan (int u, int pre)
	{
		dfn[u]=low[u]=++id;
		Stack[++top]=u;
		int child=0, flag=1;
		
		for (int i=head[u]; ~i; i=edge[i].next)
		{
			int v=edge[i].to;
			//if (v == pre) continue; //重边算一条的写法
			if (flag && v==pre)
			{//重边有效的写法
				flag = 0; 
				continue;
			}
			if (!dfn[v])
			{
				child++;
				Tarjan(v,u);
				low[u] = min(low[u], low[v]);
				if (low[v] >= dfn[u])
				{
					iscut[u] = true;  //是割点
					if (low[v]>dfn[u])
						edge[i].cut = edge[i^1].cut = true;  //是桥
				}
			}
			else low[u] = min(low[u], dfn[v]);
		}
		if (child == 1 && pre<0) //树根
			iscut[u] = false;
		if (low[u] == dfn[u])
		{
			colornum++;
			do
			{
				color[ Stack[top] ] = colornum;
			}while(Stack[top--] != u);
		}
	}

	void Init (int _n)
	{
		n=_n;
		memset(head, -1, sizeof(head));
		memset(dfn, 0, sizeof(dfn));
		memset(iscut, 0, sizeof(iscut));
		memset(color, -1, sizeof(color));
		bridgetop =e=id= 0;
		top = colornum = 0;
	}

	void Deal ()
	{
		int i;
		for (i=1; i<=n; i++) if (!dfn[i])
			Tarjan(i, -1);
		for (i=0; i<=colornum; i++)
			G[i].clear();
		bri_cut = 0;
		for (i=0; i<e; i+=2)
		{//建新图
			int u = color[edge[i].from];
			int v = color[edge[i].to];
			if (u != v)
				G[u].push_back(v), G[v].push_back(u);
			bri_cut += edge[i].cut;
		}
	}
}ob;

int n,m;
int dis[nPoint];

int BFS (int u)
{
    for (int i = 0;i<=n;i++)dis[i] = INF;
    dis[u] = 0; 
    queue<int>q;
    q.push(u);
    int pos = u, d = 0;
    while (!q.empty()){
        u = q.front(); q.pop();
        for(int i=0;i<G[u].size();i++)
        {
            int v = G[u][i];
            if(dis[v] > dis[u]+1)
            {
                dis[v] = dis[u]+1;
                q.push(v);
                if(dis[v] > d)d=dis[v] , pos = v;
            }
        }
    }
    return pos;
}

int main ()
{
#ifdef ONLINE_JUDGE
#else
	freopen("read.txt","r",stdin);
#endif
	while (scanf("%d%d",&n,&m), m+n)
	{
		ob.Init(n);
		int u,v;
		while (m--)
		{
			scanf("%d %d",&u,&v);
			ob.Add(u,v);
			ob.Add(v,u);
		}
		ob.Deal();
		u=BFS(1);
		v=BFS(u);
		printf("%d\n",ob.bri_cut - dis[v]);
	}
	return 0;
}