hdu 4612 Warm up 桥缩点

4612 Warm hdu up

题目：给出一个图，添加一条边之后，问能够在新图中得到的最少的桥的数量。

分析：我们可以双联通分量进行缩点，原图变成了一棵树。问题变成了：求树中添加一条边之后，使得不在圈的边最少。显然求一边直径，用总边数减掉最长路上的边数就是答案。注意数据存在重边的情况。

#include <set>

#include <map>

#include <list>

#include <cmath>

#include <queue>

#include <stack>

#include <string>

#include <vector>

#include <cstdio>

#include <cstring>

#include <iostream>

#include <algorithm>



using namespace std;



typedef long long ll;

typedef unsigned long long ull;



#define debug puts("here")

#define rep(i,n) for(int i=0;i<n;i++)

#define rep1(i,n) for(int i=1;i<=n;i++)

#define REP(i,a,b) for(int i=a;i<=b;i++)

#define foreach(i,vec) for(unsigned i=0;i<vec.size();i++)

#define pb push_back

#define RD(n) scanf("%d",&n)

#define RD2(x,y) scanf("%d%d",&x,&y)

#define RD3(x,y,z) scanf("%d%d%d",&x,&y,&z)

#define RD4(x,y,z,w) scanf("%d%d%d%d",&x,&y,&z,&w)

#define All(vec) vec.begin(),vec.end()

#define MP make_pair

#define PII pair<int,int>

#define PQ priority_queue

#define cmax(x,y) x = max(x,y)

#define cmin(x,y) x = min(x,y)

#define Clear(x) memset(x,0,sizeof(x))

/*



#pragma comment(linker, "/STACK:1024000000,1024000000")



int size = 256 << 20; // 256MB

char *p = (char*)malloc(size) + size;

__asm__("movl %0, %%esp\n" :: "r"(p) );



*/



char IN;

bool NEG;

inline void Int(int &x){

    NEG = 0;

    while(!isdigit(IN=getchar()))

        if(IN=='-')NEG = 1;

    x = IN-'0';

    while(isdigit(IN=getchar()))

        x = x*10+IN-'0';

    if(NEG)x = -x;

}

inline void LL(ll &x){

    NEG = 0;

    while(!isdigit(IN=getchar()))

        if(IN=='-')NEG = 1;

    x = IN-'0';

    while(isdigit(IN=getchar()))

        x = x*10+IN-'0';

    if(NEG)x = -x;

}



/******** program ********************/



const int MAXN = 200005;

const int MAXM = 1000005;



vector<int> adj[MAXN];

int po[MAXN],tol;

int low[MAXN],dfn[MAXN],tim;

bool use[MAXN];

int fa[MAXN];

int id[MAXN],ha[MAXN];



struct Edge{

    int y,id,next;

}edge[MAXM<<1];



inline void add(int x,int y,int i){

    edge[++tol].y = y;

    edge[tol].id = i;

    edge[tol].next = po[x];

    po[x] = tol;

}



int findSet(int x){

    if(x!=fa[x])

        fa[x] = findSet(fa[x]);

    return fa[x];

}



void dfs(int x,int fid){

    low[x] = dfn[x] = ++ tim;

    for(int i=po[x];i;i=edge[i].next){

        int y = edge[i].y;

        int id = edge[i].id;

        if(id==fid)continue;

        if(!dfn[y]){

            dfs(y,id);

            cmin( low[x],low[y] );

            if(low[y]<=dfn[x]){

                int px = findSet(x);

                int py = findSet(y);

                if(px!=py)fa[px] = py;

            }

        }else

            cmin( low[x],dfn[y] );

    }

}



int MAX,root;



void dfsR(int x,int sz){

    use[x] = true;

    if(sz>MAX){

        MAX = sz;

        root = x;

    }

    foreach(i,adj[x])

        if(!use[adj[x][i]])

            dfsR(adj[x][i],sz+1);

}



int main(){



#ifndef ONLINE_JUDGE

    freopen("1002.in","r",stdin);

    //freopen("sum.out","w",stdout);

#endif



    int size = 256 << 20; // 256MB

    char *p = (char*)malloc(size) + size;

    __asm__("movl %0, %%esp\n" :: "r"(p) );



    int x,y,n,m;

    while(1){

        Int(n);Int(m);

        if(!n&&!m)break;



        Clear(po);

        tol = 0;

        rep1(i,m){

            Int(x);Int(y);

            add(x,y,i);

            add(y,x,i);

        }



        rep1(i,n){

            adj[i].clear();

            fa[i] = i;

        }

        Clear(dfn);

        tim = 0;

        rep1(i,n)

            if(!dfn[i])

                dfs(i,0);



        Clear(id);

        int tot = 0;

        rep1(i,n){

            int px = findSet(i);

            if(!id[px])id[px] = ++tot;

            ha[i] = id[px];

        }

        rep1(x,n){

            int px = ha[x];

            for(int i=po[x];i;i=edge[i].next){

                int py = ha[ edge[i].y ];

                if(px==py)continue;

                adj[px].pb(py);

                adj[py].pb(px);

            }

        }



        Clear(use);

        MAX = 0;

        dfsR(1,1);



        Clear(use);

        MAX = 0;

        dfsR(root,1);



        printf("%d\n",tot-MAX);

    }



    return 0;

}