poj 2513Colored Sticks

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

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

const int sign_node=26;
const int max_node=550000;
int f[max_node];
int du[max_node];
int num=0;
int ch[max_node][sign_node];
int val[max_node];

struct Trie
{
    int sz;
    void clear(){sz=1; memset(ch[0],0,sizeof(ch[0]));}
    int idx(char c) {return c-'a';}

    int insert(char *s)
    {
        int u=0,n=strlen(s);
        for(int i=0; i<n; i++){
            int c=idx(s[i]);
            if(!ch[u][c]){
               memset(ch[sz],0,sizeof(ch[sz]));
               val[sz]=0;
               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,0,sizeof(du));
    for(int i=0; i<=max_node; i++)
      f[i]=i;
}

int main()
{
    Trie trie;
    char a[100],b[100];
    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=0,ans1=0;
    for(int i=1; i<=num; i++)
    {
        if(du[i]&1)
        ans++;
        if(ans>2||find(1)!=find(i))
        {
            printf("Impossible
");
            return 0;
        }
    }
    if(ans==1)
    printf("Impossible
");
    else
    printf("Possible
");
    return 0;
}