POJ Ikki's Story IV - Panda's Trick [2-SAT]

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
using namespace std;
const int N=1005,M=5e5;
typedef long long ll;
inline int read(){
    char c=getchar();int x=0,f=1;
    while(c<‘0‘||c>‘9‘){if(c==‘-‘)f=-1;c=getchar();}
    while(c>=‘0‘&&c<=‘9‘){x=x*10+c-‘0‘;c=getchar();}
    return x*f;
}
int n,m,a[N],b[N];
struct edge{
    int v,ne;
}e[M];
int h[N],cnt=0;
inline void ins(int u,int v){
    cnt++;
    e[cnt].v=v;e[cnt].ne=h[u];h[u]=cnt;
}
void buildGraph(){
    for(int i=1;i<=m;i++)
        for(int j=1;j<=m;j++) 
            if((a[i]<a[j]&&a[j]<b[i]&&b[i]<b[j])||(a[j]<a[i]&&a[i]<b[j]&&b[j]<b[i])){
                int x=i<<1,y=j<<1;
                ins(x,y-1);ins(y-1,x);
                ins(x-1,y);ins(y,x-1);
            }    
}
int dfn[N],low[N],dfc,belong[N],scc;
int st[N],top;
void dfs(int u){
    dfn[u]=low[u]=++dfc;
    st[++top]=u;
    for(int i=h[u];i;i=e[i].ne){
        int v=e[i].v;
        if(!dfn[v]){
            dfs(v);
            low[u]=min(low[u],low[v]);
        }else if(!belong[v])
            low[u]=min(low[u],dfn[v]);
    }
    if(dfn[u]==low[u]){
        scc++;
        while(true){
            int x=st[top--];
            belong[x]=scc;
            if(x==u) break;
        }
    }
}
bool judge(){
    for(int i=1;i<=m;i++) if(belong[i<<1]==belong[(i<<1)-1]) return false;
    return true;
}
int main(){
    freopen("in","r",stdin);
    n=read();m=read();
    for(int i=1;i<=m;i++){
        a[i]=read(),b[i]=read();
        if(a[i]>b[i]) swap(a[i],b[i]);
    }
    buildGraph();
    int _=m<<1;
    for(int i=1;i<=_;i++) if(!dfn[i]) dfs(i);
    if(judge()) puts("panda is telling the truth...");
    else puts("the evil panda is lying again");
}