poj 3207 2-sat

题目链接：http://poj.org/problem?id=3207

#include <cstdio>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <cstring>
#include <queue>
#include <vector>

#define maxn 1050
#define maxe 550 
#define INF  0x3f3f3f
using namespace std;

struct TwoSat{
    int n;
    vector<int> G[maxe*2];
    bool mark[maxe*2];
    int s[maxe*2],cnt;
    
    void init(int n){
        this->n = n;
        memset(mark,0,sizeof(mark));
        for(int i=0;i<=n*2;i++) G[i].clear();
    }
    bool dfs(int u){
        if(mark[u^1])  return false;
        if(mark[u])    return true;
        mark[u] = true;
        s[cnt++] = u;
        for(int i=0;i<G[u].size();i++){
            if(!dfs(G[u][i]))  return false;
        }
        return true;
    }
    void add_clause(int u,int uval,int v,int vval){
        u = u*2 + uval;
        v = v*2 + vval;
        G[u^1].push_back(v);
        G[v].push_back(u^1);
        G[v^1].push_back(u);
        G[u].push_back(v^1);
    }
    bool solve(){
        for(int i=0;i<n*2;i+=2){ 
            if(!mark[i] && !mark[i+1]){ 
                cnt = 0;   //这就是一直WA的原因 
                if(!dfs(i)){
                    while(cnt > 0) mark[s[--cnt]]  = false;
                    if(!dfs(i+1))  return false;  
                }
            }
        }
        return true;     //这也是WA的原因 
    }
}solver;

int main()
{
    //freopen("input.txt","r",stdin);
    int n,m;
    scanf("%d%d",&n,&m);
    struct Edge{
        int u,v;
    }e[maxe];
    for(int i=0;i<m;i++){
        int a,b;
        scanf("%d%d",&a,&b);
        if(a > b){
            int temp = a;
            a = b;
            b = temp;
        } 
        e[i].u = a;  e[i].v = b;
    }
    solver.init(m);
    for(int i=0;i<m;i++)
       for(int j=i+1;j<m;j++){
            int x1 = e[i].u ,y1 = e[i].v;
            int x2 = e[j].u ,y2 = e[j].v;
         if((x2>x1 && x2<y1 && y2 > y1) ||(y2>x1 && y2<y1 && x2<x1)){
             solver.add_clause(i,1,j,1); 
          }
       }
       if(solver.solve()) printf("panda is telling the truth...\n");
       else               printf("the evil panda is lying again\n");
}