HDU 3062 Party (2-SAT)

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <queue>
#include <stack>
using namespace std;
const int N=2009;
struct LT
{
    int to,nex;
} L[N*N];
int F
,cnt;
void add(int f,int t)
{
    L[cnt].nex = F[f];
    L[cnt].to = t;
    F[f] = cnt++;
}
int col
,low
,dfn
,post
,color,ind;
stack<int> S;
void tarjan(int k)
{
    low[k] = dfn[k] = ++ind;
    S.push(k);
    post[k] = true;
    for(int i=F[k]; i; i=L[i].nex)
    {
        int to = L[i].to;
        if(!dfn[to])
        {
            tarjan(to);
            low[k] = min(low[k],low[to]);
        }
        else if(post[to]&&dfn[to]<low[k])
            low[k] = dfn[to];
    }
    if(low[k]==dfn[k])
    {
        int i;
        color++;
        for(i=S.top(),S.pop(); i!=k; i=S.top(),S.pop())
        {
            post[i] =0,col[i] = color;
        }
        post[k]=0,col[k] = color;
    }
}
int main()
{
    freopen("in.txt","r",stdin);
    int n,m;
    while(~scanf("%d%d",&n,&m))
    {
        int a1,a2,b1,b2;
        cnt=1;
        ind = 1;
        memset(F,0,sizeof(F));
        for(int i=0; i<m; i++)
        {
            scanf("%d%d%d%d",&a1,&a2,&b1,&b2);
            add(a1+n*b1,a2+(b2^1)*n);
            add(a2+n*b2,a1+(b1^1)*n);
        }
        memset(dfn,0,sizeof(dfn));
        for(int i=0; i<n*2; i++)
            if(!dfn[i]) tarjan(i);
        bool fin = false;
        for(int i=0; i<n; i++)
            if(col[i]==col[i+n])
            {
                fin = true;
                break;
            }
        if(fin) printf("NO\n");
        else printf("YES\n");
    }
    return 0;
}