HDU 4612 桥数-缩点后树的直径

题意：

给定n个点 m条边的 无向连通图

问：

桥数-缩点后树的直径

憋了好久终于1Y了

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<vector>
#include<stdio.h>
#include<string.h>
#include<math.h>
#include<stdlib.h>
#include<iostream>
#include<queue>
using namespace std;
#define inf 233333333
#define N 200010
#define M 2000010
struct Edge{
	int from, to, nex;
	bool cut;
}edge[M];
int head[N], edgenum;
int bridgetop;
void addedge(int u, int v){
	Edge E={u,v,head[u],false};
	edge[ edgenum ] = E;
	head[u] = edgenum++;
}
int n, m;
int dfn[N], low[N], tarjan_time, tar, Stack[N], top;
int Belong[N];
bool iscut[N];
void tarjan(int u, int fa){
	dfn[u] = low[u] = ++tarjan_time;
	Stack[++top] = u;
	int child = 0, flag = 1;

	for(int i = head[u]; ~i; i = edge[i].nex)
	{
		int v = edge[i].to;
		if(flag && v==fa){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;

				if(low[v]>dfn[u])
					edge[i].cut = edge[i^1].cut = true;
			}
		}
		else low[u] = min(low[u], dfn[v]);
	}
	if(child == 1 && fa<0)iscut[u] = false;
	if(low[u] == dfn[u])
	{
		tar++;
		do
		{
			Belong[ Stack[top] ] = tar;
		}while(Stack[top--] != u);
	}
}
vector<int>G[N];
int dis[N];
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;
}
void init(){
	memset(head, -1, sizeof(head)), edgenum = 0;
	memset(dfn, 0, sizeof(dfn));
	memset(iscut, 0, sizeof(iscut));
	memset(Belong, -1, sizeof(Belong));
	bridgetop = 0;
	tarjan_time = 0;
	top = 0;
	tar = 0;
}
int main(){ 
	int i, j, u, v; 
	while(scanf("%d%d",&n,&m), m+n){
		init();
		while(m--){
			scanf("%d %d",&u,&v);
			addedge(u, v); addedge(v, u);
		}
		for(i = 1; i <= n; i++)if(!dfn[i])
			tarjan(i, -1);

		for(i = 0; i <= tar; i++)G[i].clear();
		int bri_cut = 0;
		for(i = 0; i < edgenum; i+=2 )
		{
				u = Belong[edge[i].from], v = Belong[edge[i].to];
				if(u != v)
					G[u].push_back(v), G[v].push_back(u);
				bri_cut += edge[i].cut;
		}
		u = BFS(1);
		v = BFS(u);
		printf("%d\n",bri_cut - dis[v]);
	}
	return 0;
}