hdu 4635 强连通分量+缩点

http://acm.hdu.edu.cn/showproblem.php?pid=4635

3
3 3
1 2
2 3
3 1
3 3
1 2
2 3
1 3
6 6
1 2
2 3
3 1
4 5
5 6
6 4

Case 1: -1
Case 2: 1
Case 3: 15

/**
hdu 4635  强连通分量+缩点
题目大意：给定一个图，问能最多加多少边使其还不是连通图
解题思路：http://www.xuebuyuan.com/1606580.html
*/
#include <stdio.h>
#include <string.h>
#include <algorithm>
#include <iostream>
using namespace std;
typedef long long LL;
const int maxn=100005;

struct note
{
    int v,next;
}edge[maxn*10];

int head[maxn],ip;
int st[maxn],ins[maxn],dfn[maxn],low[maxn],cnt_tar,index,top;
int in[maxn],out[maxn],num[maxn],belong[maxn];
int m,n;

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

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

void tarjan(int u)
{
    dfn[u]=low[u]=++index;
    st[++top]=u;
    ins[u]=1;
    for(int i=head[u];i!=-1;i=edge[i].next)
    {
        int v=edge[i].v;
        if(!dfn[v])
        {
            tarjan(v);
            low[u]=min(low[u],low[v]);
        }
        else if(ins[v])
        {
            low[u]=min(low[u],dfn[v]);
        }
    }
    if(dfn[u]==low[u])
    {
        cnt_tar++;
        int j;
        do
        {
            j=st[top--];
            ins[j]=0;
            belong[j]=cnt_tar;
            num[cnt_tar]++;
        }while(j!=u);
    }
}

void solve()
{
    top=0,index=0,cnt_tar=0;
    memset(dfn,0,sizeof(dfn));
    memset(low,0,sizeof(low));
    for(int i=1;i<=n;i++)
    {
        if(!dfn[i])
            tarjan(i);
    }
}

int main()
{
    int T,tt=0;
    scanf("%d",&T);
    while(T--)
    {
        scanf("%d%d",&n,&m);
        init();
        for(int i=1;i<=m;i++)
        {
            int u,v;
            scanf("%d%d",&u,&v);
            addedge(u,v);
        }
        memset(num,0,sizeof(num));
        solve();
        if(cnt_tar==1)
        {
            printf("Case %d: -1\n",++tt);
            continue;
        }
        memset(in,0,sizeof(in));
        memset(out,0,sizeof(out));
        for(int u=1;u<=n;u++)
        {
            for(int i=head[u];i!=-1;i=edge[i].next)
            {
                int v=edge[i].v;
                if(belong[u]==belong[v])continue;
                out[belong[u]]++;
                in[belong[v]]++;
            }
        }
        LL all=(LL)n*(n-1)-m;
        LL ans=0;
        for(int i=1;i<=cnt_tar;i++)
        {
            if(in[i]==0||out[i]==0)
            {
                ans=max(ans,all-(LL)num[i]*(n-num[i]));
            }
        }
        printf("Case %d: %I64d\n",++tt,ans);
    }
    return 0;
}

hdu 4635 强连通分量+缩点

原文：http://blog.csdn.net/lvshubao1314/article/details/43955231