poj 3207 2-sat

#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");
}