hdu 1824 Let's go home (2-sat)

# include 
# include 
# include 
# include 
using namespace std;
# define MAXN  6010
# define MAXM 10010
struct node
{
    int to;
    int next;
} edge[MAXM];
int tot;
int tim,num;
int be[MAXN];
bool instack[MAXN];
int dfn[MAXN],low[MAXN];
int head[MAXN];
int pos[MAXN];
stacks;
int n,m;
void init()
{
    tot=0;
    memset(head,-1,sizeof(head));
}
void add(int from,int to)
{
    edge[tot].to=to;
    edge[tot].next=head[from];
    head[from]=tot++;
}
void tarjan(int cur)
{
    dfn[cur]=low[cur]=++tim;
    s.push(cur);
    instack[cur]=true;
    for(int i=head[cur]; i!=-1; i=edge[i].next)
    {
        int to=edge[i].to;
        if(!dfn[to])
        {
            tarjan(to);
            low[cur]=min(low[cur],low[to]);
        }
        else if(instack[to])
        {
            low[cur]=min(low[cur],dfn[to]);
        }
    }
    if(dfn[cur]==low[cur])
    {
        int tem = -1;
        ++num;
        while(tem != cur)
        {
            tem = s.top();
            s.pop();
            instack[tem] = false;
            be[tem] = num;
        }
    }
}

void solve()
{
    memset(dfn,0,sizeof(dfn));
    memset(low,0,sizeof(low));
    memset(instack,0,sizeof(instack));
    memset(be,0,sizeof(be));
    while(!s.empty())
        s.pop();
    tim=num=0;
    for(int i=0; i<2*n; i++)
    {
        if(!dfn[i])
            tarjan(i);
    }
    bool ans=true;
    for(int i=0; i<2*n; i++)
    {
        if(be[i]==be[i+n])
        {
            ans=false;
            break;
        }
    }
    if(ans)
        puts("yes");
    else
        puts("no");
}
int main()
{
    int x,y,z;
    while(~scanf("%d%d",&n,&m))
    {
        init();
        for(int i=0; i=n)
                add(pos[x]-n,pos[y]);
            else
                add(pos[x]+n,pos[y]);

            if(pos[y]>=n)
                add(pos[y]-n,pos[x]);
            else
                add(pos[y]+n,pos[x]);

        }
        solve();
    }
    return 0;
}