poj 3207 2-SAT

题意：0-n-1围成一个环，给M条边，可以从外面连，也可以从内部连，问连玩之后能不想交的方案。

解法：2-sat，合理匹配方案

i代表从内部连接第i条边，i+m代表从外部连接第I条边；

如果C(i)=i+j，那么需要增加弧（-i,j）,(-j,i),判断连通分量，如果选中连通分量中的一个点，那么需要选择连通分量中的所有点，如果c和c(i)，在同一个连通分量内，则产生矛盾，此2-SAT无解。

此题只需要判断有无解就好了。

其实，我也不是很懂，多刷题吧。

/*
----------------------------------

Love is more than a word.
It says so much.
When I see these four letters,
I almost feel your touch.
This is only happened since
I fell in love with you.
Why this word does this,
I haven't got a clue.

To My Goddess
CY
----------------------------------
*/

#include<iostream>
#include<cstring>
#include<algorithm>
#include<cstdlib>
#include<vector>
#include<cmath>
#include<stdlib.h>
#include<iomanip>
#include<list>
#include<deque>
#include<map>
#include <stdio.h>
#include <queue>

#define maxn 500000+5

#define ull unsigned long long
#define ll long long
#define reP(i,n) for(i=1;i<=n;i++)
#define REP(i,a,b) for(i=a;i<=b;i++)
#define rep(i,n) for(i=0;i<n;i++)

#define cle(a) memset(a,0,sizeof(a))
#define clehead(a) rep(i,maxn)a[i]=-1

/*
The time of story :
**  while(1)
{
once upon a time,
there was a mountain,
on top of which there was a temple,
in which there was an old monk and a little monk.
Old monk was telling stories inside the temple.
What was he talking about?
**  }

ÎûÎû
(*^__^*)
*/

#define sci(a) scanf("%d",&a)
#define scd(a) scanf("%lf",&a)
#define pri(a) printf("%d",a)
#define prie(a) printf("%d\n",a)
#define prd(a)  printf("%lf",a)
#define prde(a) printf("%lf\n",a)
#define pre printf("\n")

#define LL(x) x<<1
#define RR(x) x<<|1

#define pb push_back
#define mod 90001
#define PI 3.141592657

const ull INF = 1LL << 61;
const int inf =   int(1e5)+10;
const double eps=1e-5;

using namespace std;

struct node
{
int be;
int to,w;
int next;
}edge[maxn];

bool cmp(int a,int b){
return a>b;
}
node cop[maxn];
int head[maxn];
int m;
int low[maxn],dfn[maxn],stack[maxn],used[maxn];
int cnt=0;
int in[maxn],out[maxn];
void add(int x,int y)
{
edge[cnt].be=x;
edge[cnt].to=y;
edge[cnt++].next=head[x];
head[x]=(cnt-1);
}
void init()
{
memset(head,-1,sizeof(head));
cle(used),cle(low),cle(dfn),cle(in),cle(out);
m=0;
cnt=0;
}
int tarbfs(int k,int lay,int &scc_num)
{
used[k]=1;
low[k]=lay;
dfn[k]=lay;
stack[++m]=k;
for(int i=head[k];i!=-1;i=edge[i].next)
{
if(used[edge[i].to]==0)
{
tarbfs(edge[i].to,++lay,scc_num);
}
if(used[edge[i].to]==1)
{
low[k]=min(low[k],low[edge[i].to]);
}
}
if(dfn[k]==low[k])
{
++scc_num;
do{
low[stack[m]]=scc_num;
used[stack[m]]=2;
}while(stack[m--]!=k);
}
return 0;
}
int main()
{
freopen("in.txt","r",stdin);
//freopen("out.txt","w",stdout);
int n,m;
while(scanf("%d%d",&n,&m)!=EOF)
{
int i,j,k;
init();
rep(i,m)
{
scanf("%d%d",&cop[i].be,&cop[i].to);
if(cop[i].be>cop[i].to)swap(cop[i].be,cop[i].to);
}
rep(i,m)
{
REP(j,i+1,m-1)
{
if(cop[i].to>cop[j].be&&cop[i].to<cop[i].to&&cop[i].be<cop[j].be)
{
add(i+m,j),add(j,i+m),add(j+m,i),add(i,j+m);
}
if(cop[i].to>cop[j].to&&cop[i].be>cop[j].be&&cop[i].be<cop[j].to)
{
add(i+m,j),add(j,i+m),add(j+m,i),add(i,j+m);
}
}
}
int scc_num=0;
int lay=1;
rep(i,2*m)
{
if(used[i]==0)
{
tarbfs(i,lay,scc_num);
}
}
bool flag=true;
rep(i,m){
if(low[i]==low[i+m]){
flag=false;
break;
}
}
if(flag)
{
printf("panda is telling the truth...\n");
}
else {
printf("the evil panda is lying again\n");
}
}
return 0;
}