hdu 1116

欧拉回路，利用并查集来实现；

代码：

#include<cstdio>
#include<cstring>
#include<vector>
using namespace std;
int a[30],f[30],b[30];
bool vis[30];

int find(int x)
{
    return f[x]==-1?x:f[x]=find(f[x]);
}

void combine(int x,int y)
{
    int n=find(x);
    int m=find(y);
    if(n!=m) f[n]=m;
}

bool oula()
{
    int st=-1;
    for(int i=0; i<26; i++)
    {
        if(a[i]||b[i])
            if(st==-1)st=find(i);
            else if(st!=find(i)) return false;
    }
    vector<int>v;
    for(int i=0; i<26; i++)
        if(a[i]!=b[i])
            v.push_back(a[i]-b[i]);
    return v.size()==0||v.size()==2&&v[1]*v[0]==-1;
}

int main()
{
    int t,n;
    char s[1009];
    scanf("%d",&t);
    while(t--)
    {
        memset(a,0,sizeof a);
        memset(f,-1,sizeof f);
        memset(b,0,sizeof b);
        int ans=0,cnt=0;
        bool flag=1;
        scanf("%d",&n);
        for(int i=0; i<n; i++)
        {
            scanf("%s",&s);
            int l=strlen(s);
            int x=s[0]-'a',y=s[l-1]-'a';
            a[x]++,b[y]++;
            combine(x,y);
        }
        if(oula()) printf("Ordering is possible.
");
        else puts("The door cannot be opened.");
    }
    return 0;
}