【2-SAT】【tarjan】【建图】POJ 3207 Ikki's Story IV - Panda's Trick

【题目】http://poj.org/problem?id=3207

【大意】在圆上取任意点两两连线，线都在圆内部或外部。问能否使这些线不交叉。

【思路】

【参考】https://blog.csdn.net/qq_30974369/article/details/73930235

【代码】

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<iostream>
using namespace std;
const int M=200005;

int n,m;
int linkx[M],linky[M];

int head[M];
class node
{
public:
    int v,next;
};
node edge[M];
int ei;
int Stack[M],si;
int DFN[M],LOW[M],tot;
int inthestack[M];
int scc;
int belong[M];

void tarjan(int x)
{
    Stack[++si]=x;
    inthestack[x]=1;
    DFN[x]=LOW[x]=++tot;
    for(int i=head[x]; i!=-1; i=edge[i].next)
    {
        int& t=edge[i].v;
        if(!DFN[t])
        {
            tarjan(t);
            LOW[x]=min(LOW[x],LOW[t]);
        }
        else if(DFN[t]&&inthestack[t])
        {
            LOW[x]=min(LOW[x],DFN[t]);
        }
    }
    if(DFN[x]==LOW[x])
    {
        scc++;
        do
        {
            belong[Stack[si]]=scc;
            inthestack[Stack[si]]=0;
            si--;
        }
        while(x!=Stack[si+1]);
    }
    return;
}

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

void init()
{
    memset(head,-1,sizeof(head));
    memset(inthestack,0,sizeof(inthestack));
    memset(LOW,0,sizeof(LOW));
    memset(DFN,0,sizeof(DFN));
    scc=tot=ei=si=0;
    return;
}

void makeedge()
{
    for(int i=1; i<=m; i++)
    {
        for(int j=i+1; j<=m; j++)
        {
            if((linkx[i]<linkx[j]&&linky[i]>linkx[j]&&
                    linky[i]<linky[j])||
                    (linkx[j]<linkx[i]&&linky[j]>linkx[i]&&
                     linky[j]<linky[i])
              )
            {
                addedge(i,j+m);
                addedge(j+m,i);
                addedge(i+m,j);
                addedge(j,i+m);
            }
        }
    }
}



int main()
{
    init();
    scanf("%d%d",&n,&m);
    for(int i=1; i<=m; i++)
    {
        int a,b;
        scanf("%d%d",&a,&b);
        if(a>b)
            swap(a,b);
        a++,b++;
        linkx[i]=a,linky[i]=b;
    }
    makeedge();
    for(int i=1; i<=2*m; ++i) //缩点
        if(!DFN[i])
            tarjan(i);
    for(int i=1; i<=m; i++)
    {
        if(belong[i]==belong[i+m])
        {
            cout<<"the evil panda is lying again"<<endl;
            return 0;
        }
    }
    cout<<"panda is telling the truth..."<<endl;
    return 0;
}