hoj 13788 Dwarves

//首先判环我都不会 dfs实在写的戳 后来测了好多数据发现dfs写错了 唉 其次不会哈希啊 这个哈希还是dalao手写给我的 珍惜！！！
#include <iostream>
#include <algorithm>
#include <stdio.h>
#include <vector>
#include <string>
#include <string.h>
#include <queue>
#include <math.h>
#include <map>
#define ULL unsigned long long
using namespace std;
const int maxn=10010;
const int INF=0x3f3f3f3f;
#define mod 10007
int cnt;
vector <int> e[10010];

//map<ULL,int> mp;
int f[10010];
int de[maxn];
int tot;
bool flag=true;
void dfs(int now){
    f[now]=-1;
    for(int i=0;i<e[now].size();i++)
    {
        if(f[e[now][i]]==-1) {flag=false;}
        else if(f[e[now][i]]==0) dfs(e[now][i]);
    }
    f[now]=1;
}

const int maxbuf = 11111;
struct node {
    ULL key;
    int id,next;
}buf[maxbuf];
int h[maxn];
ULL fuck(char a[30]){
    ULL ha=0;
    for(int i=0;a[i];i++){
        ha*=52;
        ha+=a[i];
    }
    return (ha);
}
void inithash() {
    tot = 0;
    memset(h,-1,sizeof(h));
}
int ins(ULL key) {
    int now = ++tot;
    buf[now].key = key, buf[now].id = now, buf[now].next = h[key%mod];
    h[key%mod] = now;
    return now;
}
int fnd(ULL key) {
    for (int i=h[key%mod];~i;i=buf[i].next) {
        if (buf[i].key == key) return buf[i].id;
    }
    return 0;
}

char s1[30],s2[30];
int main(){
    #ifdef shuaishuai
    freopen("C:\Users\hasee\Desktop\trianglen=100000.in","r",stdin);
    // freopen("C:\Users\hasee\Desktop\b.txt","w",stdout);
    // freopen("C:\Users\hasee\Desktop\picture.in","r",stdin);
#endif
   // ios::sync_with_stdio(false);
    char s;
    int n;
        memset(f,0,sizeof f);
        memset(de,0,sizeof de);
        inithash();
    while(scanf("%d
",&n)!=EOF)
    {
        cnt=1;

        for(int i=0;i<n;i++)
        {
           // cout<<"@"<<endl;
            scanf("%s %c %s
",s1,&s,s2);
//            cout<<s1<<" "<<s<<" "<<s2<<endl;

            ULL x1=fuck(s1);
            ULL x2=fuck(s2);
            int u = fnd(x1), v = fnd(x2);

//            cout<<"before  "<<u<<" "<<v<<" "<<tot<<endl;
//            cout<<x1<<" "<<x2<<endl;
            if (!u) u = ins(x1);
            if (!v) v = ins(x2);
//            printf("%s %d %s %d
",s1,u,s2,v);
//            cout<<u<<" "<<v<<" "<<tot<<endl;
           // printf("%d %d
",x1,x2);
 //           int x1=mp[t1],x2=mp[t2];
//            if(x1==0)x1=mp[t1]=cnt++;
//            if(x2==0)x2=mp[t2]=cnt++;
            if(s=='>')
            {
              e[u].push_back(v);
              de[v]++;
            }
            else
            {
               e[v].push_back(u);
               de[u]++;
            }
        }
        for(int i=1;i<=tot;i++  ){
            if(f[i]==0) dfs(i);
            if(!flag) break;
        }
        if(flag)
             printf("possible
");
        else
             printf("impossible
");
             for(int i=0;i<=tot;i++)e[i].clear(),f[i]=0,h[i]=-1,de[i]=0;
             tot=0;
    }
    return 0;
}