poj 2513Colored Sticks

http://poj.org/problem?id=2513

 #include<cstdio>
 #include<cstdlib>
 #include<cstring>
 using namespace std;

 const int sign_node=;
 const int max_node=;
 int f[max_node];
 int du[max_node];
 int num=;
 int ch[max_node][sign_node];
 int val[max_node];

 struct Trie
 {
     int sz;
     void clear(){sz=; memset(ch[],,sizeof(ch[]));}
     int idx(char c) {return c-'a';}

     int insert(char *s)
     {
         int u=,n=strlen(s);
         for(int i=; i<n; i++){
             int c=idx(s[i]);
             if(!ch[u][c]){
                memset(ch[sz],,sizeof(ch[sz]));
                val[sz]=;
                ch[u][c]=sz++;
             }
             u=ch[u][c];
         }
         if(!val[u]) val[u]=++num;
         return val[u];
     }
 };

 int find(int x)
 {
     if(x!=f[x])
     f[x]=find(f[x]);
     return f[x];
 }

 void merge(int x,int y)
 {
     int fx=find(x);
     int fy=find(y);
     if(fx!=fy){
         f[fx]=fy;
     }
 }

 void inti()
 {
     memset(du,,sizeof(du));
     for(int i=; i<=max_node; i++)
       f[i]=i;
 }

 int main()
 {
     Trie trie;
     char a[],b[];
     inti();
     trie.clear();
     while(scanf("%s%s",a,b)!=EOF)
     {
         int id1=trie.insert(a);
         int id2=trie.insert(b);
         du[id1]++;
         du[id2]++;
         merge(id1,id2);
     }
     int ans=,ans1=;
     for(int i=; i<=num; i++)
     {
         if(du[i]&)
         ans++;
         if(ans>||find()!=find(i))
         {
             printf("Impossible\n");
             return ;
         }
     }
     if(ans==)
     printf("Impossible\n");
     else
     printf("Possible\n");
     return ;
 }