[{"sectionTitle":"题目描述","type":"Text","text":"来自 ICPC CERC 1999/2000，有改动。\r\n\r\n有 $N$ 个盘子，每个盘子上写着一个仅由小写字母组成的英文单词。你需要给这些盘子安排一个合适的顺序，使得相邻两个盘子中，前一个盘子上单词的末字母等于后一个盘子上单词的首字母。请你编写一个程序，判断是否能达到这一要求。如果能，请给出一个合适的顺序。","subType":"markdown"},{"sectionTitle":"输入格式","type":"Text","text":"多组数据。第一行给出数据组数 $T$，每组数据第一行给出盘子数量 $N$，接下去 $N$ 行给出小写字母字符串，一种字符串可能出现多次。","subType":"markdown"},{"sectionTitle":"输出格式","type":"Text","text":"若存在一组合法解输出Ordering is possible.，否则输出The door cannot be opened.。","subType":"markdown"},{"sectionTitle":"样例","type":"Sample","text":"","subType":"markdown","payload":["3\n2\nacm\nibm\n3\nacm\nmalform\nmouse\n2\nok\nok","The door cannot be opened.\nOrdering is possible.\nThe door cannot be opened."]},{"sectionTitle":"数据范围与提示","type":"Text","text":"$1 \\le N \\le 10^5,|S| \\le 1000$","subType":"markdown"}]