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Warm up

4 4
1 2
1 3
1 4
2 3
0 0 

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#include <cstdio>
#include <cstring>
#include <algorithm>
#include <queue>
#include <stack>
#define maxn 200000+1000
#define maxm 2000000+1000
#pragma comment(linker, "/STACK:1024000000,1024000000")
using namespace std;
int n ,m;

struct node {
    int u, v, next;
};

node edge[maxm];
int head[maxn], cnt;
int dfn[maxn], low[maxn];
int Stack[maxn], top;
bool Instack[maxn];
int Belong[maxn];
int dfs_clock;
int ebc_clock;
int bridge;
vector<int>Map[maxn];
int dist[maxn], vis[maxn];
int ans, nod;

void init(){
    cnt = 0;
    memset(head, -1, sizeof(head));
}

void addedge(int u, int v){
    edge[cnt] = {u, v, head[u]};
    head[u] = cnt++;
}

void getmap(){
    while(m--){
        int a, b;
        scanf("%d%d", &a, &b);
        addedge(a, b);
        addedge(b, a);
    }
}
void tarjan(int u, int per){
    int v;
    low[u] = dfn[u] = ++dfs_clock;
    Stack[top++] = u;
    Instack[u] = true;
    int have = 1;
    for(int i = head[u]; i != -1; i = edge[i].next){
        v = edge[i].v;
        if(v == per && have){
            have = 0;
            continue;
        }
        if(!dfn[v]){
            tarjan(v, u);
            low[u] = min(low[u], low[v]);
            if(low[v] > dfn[u])
                bridge++;
        }
        else if(Instack[v])
            low[u] = min(low[u], dfn[v]);
    }
    if(dfn[u] == low[u]){
        ebc_clock++;
        do{
            v = Stack[--top];
            Instack[v] = false;
            Belong[v] = ebc_clock;
        }while(v != u);
    }
}

void suodian(){//Ëõµã½¨ÐÂͼ
    for(int i = 1; i <= ebc_clock; ++i)
        Map[i].clear();
    for(int i = 0; i < cnt; i = i + 2){
        int u = Belong[edge[i].u];
        int v = Belong[edge[i].v];
        if(u != v){
            Map[u].push_back(v);
            Map[v].push_back(u);
        }
    }
}

void find(){
    memset(low, 0, sizeof(low));
    memset(dfn, 0, sizeof(dfn));
    memset(Instack, false, sizeof(Instack));
    memset(Belong, 0, sizeof(Belong));
    dfs_clock = ebc_clock = top = bridge = 0;
    for(int i = 1; i <= n; ++i){
        if(!dfn[i])
            tarjan(i, i);
    }
}

void BFS(int x){
    queue<int>q;
    memset(vis, 0, sizeof(vis));
    memset(dist, 0, sizeof(dist));
    vis[x] = 1;
    ans = 0;
    nod = x;
    q.push(x);
    while(!q.empty()){
        int u = q.front();
        q.pop();
        for(int i = 0; i < Map[u].size(); ++i){
            int v = Map[u][i];
            if(!vis[v]){
                vis[v] = 1;
                dist[v] = dist[u] + 1;
                if(ans < dist[v]){
                    nod = v;
                    ans = dist[v];
                }
                q.push(v);
            }
        }
    }
}

int main (){
    while(scanf("%d%d", &n, &m), n || m){
        init();
        getmap();
        find();
        suodian();
        BFS(1);
        BFS(nod);
        printf("%d\n", bridge - ans);
    }
    return 0;
}

HDU 4612--Warm up 【无向图边双连通求桥数 && 缩点后重建图求树的直径】

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原文地址：http://blog.csdn.net/hpuhjh/article/details/47760119