UVA

Play on Words

Some of the secret doors contain a very interesting word puzzle. The team of archaeologists has to solve it to open that doors. Because there is no other way to open the doors, the puzzle is very important for us.

There is a large number of magnetic plates on every door. Every plate has one word written on it. The plates must be arranged into a sequence in such a way that every word begins with the same letter as the previous word ends. For example, the word ``acm'' can be followed by the word ``motorola''. Your task is to write a computer program that will read the list of words and determine whether it is possible to arrange all of the plates in a sequence (according to the given rule) and consequently to open the door.

Input Specification

The input consists of T test cases. The number of them (T) is given on the first line of the input file. Each test case begins with a line containing a single integer number Nthat indicates the number of plates (1 <= N <= 100000). Then exactly Nlines follow, each containing a single word. Each word contains at least two and at most 1000 lowercase characters, that means only letters 'a' through 'z' will appear in the word. The same word may appear several times in the list.

Output Specification

Your program has to determine whether it is possible to arrange all the plates in a sequence such that the first letter of each word is equal to the last letter of the previous word. All the plates from the list must be used, each exactly once. The words mentioned several times must be used that number of times.

If there exists such an ordering of plates, your program should print the sentence "Ordering is possible.". Otherwise, output the sentence "The door cannot be opened.".

Sample Input

32acmibm3acmmalformmouse2okok

Output for the Sample Input

The door cannot be opened.Ordering is possible.The door cannot be opened.

因为前几天研究过并查集，所以打算用并查集试一下，DFS的做法，稍后研究。

#include <bits/stdc++.h>
#define maxn 26
using namespace std;

char word[1024];
int in[maxn], out[maxn], pre[maxn];
bool top[maxn];

void init()
{
    memset(in, 0, sizeof(in));
    memset(out, 0, sizeof(out));
    memset(top, false, sizeof(top));
    for (int i = 0; i < maxn; i++)
        pre[i] = i;
}

int Find(const int x)
{
    int l = x;
    while (l != pre[l])
        l = pre[l];
	pre[x] = l;
    return l;
}

void Join(const int x, const int y)
{
    int fx = Find(x), fy = Find(y);
    if (fx != fy)
        pre[fx] = fy;
}

int get_top()
{
    int cnt = 0;
    for(int i = 0; i < maxn; i++)
        if((in[i] || out[i]) && pre[i] == i)
            cnt++, top[i] = true;
    return cnt;
}

int main()
{
    int T; cin >> T;
    while(init(), T--){
        int N; cin >> N;
        bool ok = true;
        for(int i = 0; i < N; i++){
            cin >> word;
            in[word[0]-'a']++;
            out[word[strlen(word)-1]-'a']++;
            Join(word[0]-'a', word[strlen(word)-1]-'a');
        }
        if(get_top()!=1) ok = false;
        if(ok){
            int cnt1 = 0, cnt2 = 0;
            for(int i = 0; i < maxn; i++)
                if(in[i] != out[i]){
                    if(in[i] == out[i] + 1) cnt1++;
                    else if(in[i] == out[i] - 1) cnt2++;
                    else{
                        ok = false;
                        break;
                    }
                }
            if(!((cnt1==0&&cnt2==0) || (cnt1==1&&cnt2==1)))
                ok = false;
        }
        if(ok) puts("Ordering is possible.");
        else puts("The door cannot be opened.");
    }
    return 0;
}