hud1116

和poj2337類似，不需要輸出路徑，需要注意的是要記錄單詞首尾字母所對應的頂點，並非在一個case中26個字母都對應是頂點。

#include 
#include 
#define maxn 26

int tree[maxn];
int degree[maxn];
int vis[maxn];
int findRoot(int u){
	if(tree[u] == -1)	return u;
	int v = findRoot(tree[u]);
	tree[u] = v;
	return v;
}

int main(){
	int t, n;
	char word[1001];
	scanf("%d", &t);
	while(t--){
		scanf("%d", &n);
		memset(tree, -1, sizeof(tree));
		memset(degree, 0, sizeof(degree));
		memset(vis, 0, sizeof(vis));
		while(n--){
			scanf("%s", word);
			int start = word[0] - 'a', end = word[strlen(word) - 1] - 'a';
			--degree[start];	++degree[end];
			vis[start] = 1; vis[end] = 1;
			int p = findRoot(start), q = findRoot(end);
			if(p != q)	tree[p] = q;
		}
		int i, oth = 0, pos = 0, neg = 0, c = 0;
		for(i = 0; i < maxn; ++i){
			if(tree[i] == -1 && vis[i])	++c;
			if(degree[i] == 1)	++pos;
			else	if(degree[i] == -1)	++neg;
			else	if(degree[i] != 0)	++oth;
		}
		if(c==1 && oth==0 && (neg==0&&pos==0 || neg==1&&pos==1))
			printf("Ordering is possible.\n");
		else
			printf("The door cannot be opened.\n");
	}
	return 0;
}