hdu 3062 Party

#include<map>
#include<set>
#include<stack>
#include<queue>
#include<cmath>
#include<vector>
#include<cstdio>
#include<string>
#include<cstring>
#include<cstdlib>
#include<iostream>
#include<algorithm>
#define  inf 0x0f0f0f0f
using namespace std;

struct SCC
{
    static const int maxn=2000+10;
    vector<int>group[maxn],scc[maxn];
    int pre[maxn],lowlink[maxn],sccno[maxn],dfs_clock,scc_cnt,n,m;
    stack<int>S;

    void init()
    {
        for (int i=0;i<=n;i++) group[i].clear();
    }

    void addedge(int from,int to)
    {
        group[from].push_back(to);
    }

    void dfs(int u)
    {
        pre[u]=lowlink[u]=++dfs_clock;
        S.push(u);
        for (int i=0;i<group[u].size();i++)
        {
            int v=group[u][i];
            if (!pre[v])
            {
                dfs(v);
                lowlink[u]=min(lowlink[u],lowlink[v]);
            }
            else if (!sccno[v])
            {
                lowlink[u]=min(lowlink[u],pre[v]);
            }
        }
        if (lowlink[u]==pre[u])
        {
            scc_cnt++;
            scc[scc_cnt].clear();
            while (1)
            {
                int x=S.top();
                S.pop();
                scc[scc_cnt].push_back(x);
                sccno[x]=scc_cnt;
                if (x==u) break;
            }
        }
    }

    void find_scc()
    {
        dfs_clock=scc_cnt=0;
        memset(pre,0,sizeof(pre));
        memset(sccno,0,sizeof(sccno));
        for (int i=1;i<=n;i++)
        if (!pre[i]) dfs(i);
    }
};

SCC people;

int main()
{
    int N,M,x,y,x1,y1;
    while(scanf("%d",&N)!=EOF)
    {
        people.n=2*N;
        people.init();
        scanf("%d",&M);
        for (int i=1;i<=M;i++)
        {
            scanf("%d%d%d%d",&x,&y,&x1,&y1);
            people.addedge(x*2+x1,(y*2+y1)^1);
            people.addedge(y*2+y1,(x*2+x1)^1);
        }
        people.find_scc();
        bool flag=true;
        for (int i=0;i<2*N;i+=2)
        {
            if (people.sccno[i]==people.sccno[i+1])
            {
                flag=false;
                break;
            }
        }
        if (flag) printf("YES\n");
        else printf("NO\n");
    }
    return 0;
}