HDU 4612——Warm up——————【边双连通分量、树的直径】

#include<stdio.h>
#include<string.h>
#include<algorithm>
#include<vector>
#include<stack>
using namespace std;
const int maxn = 200100;
struct Edge{
    int from,to,dist,next;
    Edge(){}
    Edge(int _to,int _next):to(_to),next(_next){}
}edges[maxn*10];
int head[maxn], tot;
int dfs_clock, dfn[maxn], brinum;
int Stack[maxn], instack[maxn], top, ebccno[maxn], ebcc_cnt;
int deg[maxn];
vector<int>G[maxn];
void init(){
    tot = 0;
    brinum = dfs_clock = 0;
    top = 0;
    ebcc_cnt = 0;
    memset(deg,0,sizeof(deg));
    memset(head,-1,sizeof(head));
}
void AddEdge(int _u,int _v){
    edges[tot] = Edge(_v,head[_u]);
    head[_u] = tot++;
}
int dfs(int u,int fa){  
    int lowu = dfn[u] = ++dfs_clock;
    Stack[++top] = u;
   // instack[u] = 1;
    for(int i = head[u]; i != -1; i = edges[i].next){
        int v = edges[i].to;
        if(!dfn[v]){
            int lowv = dfs(v,i);
            lowu = min(lowu,lowv);
            if(lowv > dfn[u]){
                brinum++;
            }
        }else if(dfn[v] < dfn[u] && (fa^1) != i){//这里用边的编号来标记是否是同一条边的回边
            lowu = min(lowu,dfn[v]);
        }
    }
    if(dfn[u] == lowu){     //找到一个边双连通分量
        ebcc_cnt++;
        for(;;){
            int v = Stack[top--];
         //   instack[v] = 0;
            ebccno[v] = ebcc_cnt;   //给每个点划分一个分量标号
            if(u == v){
                break;
            }
        }
    }
  //  low[u] = lowu;
    return lowu;
}
void find_ebcc(int n){
    memset(dfn,0,sizeof(dfn));
    memset(instack,0,sizeof(instack));
    for(int i = 1; i <= n; i++){
        if(!dfn[i]){
            dfs(i,-1);
        }
    }
}
int pos, Maxd;
void dfs1(int u,int dep,int fa){    //求树的直径
    if(dep > Maxd){
        Maxd = dep;
        pos = u;
    }
    for(int i = 0; i < G[u].size(); i++){
        int v = G[u][i];
        if(fa == v){ continue; }
        dfs1(v,dep+1,u);
    }
}
int main(){
    int n,m;
    while(scanf("%d%d",&n,&m)!=EOF&&(n+m)){
        init();
        for(int i = 0; i <= n; i++){
            G[i].clear();
        }
        int a,b;
        for(int i = 0; i < m; i++){
            scanf("%d%d",&a,&b);
            AddEdge(a,b);
            AddEdge(b,a);
        }
        find_ebcc(n);
        for(int i = 1; i <= n; i++){
            for(int j = head[i]; j != -1; j = edges[j].next){
                int v = edges[j].to;
                if(ebccno[i] != ebccno[v]){     //重新构图，形成树
                    G[ebccno[i]].push_back(ebccno[v]);
                }
            }
        }
        pos = 1, Maxd = 0;
        dfs1(1,0,-1);
        int st = pos; Maxd = 0;
        dfs1(pos,0,-1);
        printf("%d\n",brinum - Maxd);
    }
    return 0;
}