Uva10972(RevolC FaeLoN)

Posted 2020-07-12 Kurokey

题目链接:传送门

题目大意:给你一副无向图，问至少加多少条边使图成为边双联通图

题目思路:tarjan算法+缩点（如果已经是双连通图就直接输出0）

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <cstring>
#include <stack>
#include <cctype>
#include <queue>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <climits>
#define lson root<<1,l,mid
#define rson root<<1|1,mid+1,r
#define fi first
#define se second
#define ping(x,y) ((x-y)*(x-y))
#define mst(x,y) memset(x,y,sizeof(x))
#define mcp(x,y) memcpy(x,y,sizeof(y))
#define Min(x,y) (x<y?x:y)
#define Max(x,y) (x>y?x:y)
using namespace std;
#define gamma 0.5772156649015328606065120
#define MOD 1000000007
#define inf 0x3f3f3f3f
#define N 11005
#define maxn 5005
typedef long long LL;
typedef pair<int,int> PII;

stack<int>sk;
int n,m,hcnt,deep,block,leaf;
int d[maxn],dfn[maxn],low[maxn],head[maxn];
int color[maxn],vis[maxn];
struct Node{
    int to,next;
    Node(){}
    Node(int a,int b):to(a),next(b){}
}node[N<<1];

inline void add(int x,int y){
    node[hcnt]=Node(y,head[x]);
    head[x]=hcnt++;
}

inline void init(){
    while(!sk.empty())sk.pop();
    mst(vis,0);
    mst(head,-1);
    mst(d,0);
    mst(dfn,0);
    hcnt=deep=0;
    block=leaf=0;
}

void dfs(int x,int fa){
    int flag=1;  ///这个不是必要的,有重边时有效
    sk.push(x);
    vis[x]=1;
    low[x]=dfn[x]=++deep;
    for(int i=head[x];~i;i=node[i].next){
        int e=node[i].to;
        if(e==fa&&flag){flag=0;continue;}
        if(vis[e])low[x]=min(low[x],dfn[e]);
        else if(!dfn[e]){
            dfs(e,x);
            low[x]=min(low[x],low[e]);
        }
    }
    if(low[x]==dfn[x]){
        ++block;    ///缩点
        int co;
        do{
            co=sk.top();sk.pop();
            vis[co]=0;
            color[co]=block;
        }while(co!=x);
    }
}

int main(){
    int i,j,group,Case=0,x,y;
    while(scanf("%d%d",&n,&m)!=EOF){
        init();
        for(i=0;i<m;++i){
            scanf("%d%d",&x,&y);
            add(x,y);
            add(y,x);
        }
        for(i=1;i<=n;++i){
            if(!dfn[i])dfs(i,-1);
        }
        if(block==1){printf("0\n");continue;}
        for(i=1;i<=n;++i)
        for(j=head[i];~j;j=node[j].next){
            int e=node[j].to;
            if(color[i]!=color[e])
                ++d[color[i]];   ///d数组保留的是缩点后每个点的度数
        }
        for(i=1;i<=block;++i){
            if(d[i]==0)leaf+=2;  ///如果有孤立的点,叶子节点加2
            else if(d[i]==1)++leaf;
        }
        printf("%d\n",(leaf+1)/2);
    }
    return 0;
}