sicily 1198题 给出N个字符串，求这几个字符串组成的序列中字典序最小序列-Descr iption

Dr lee cuts a string S into N pieces,s[1],…,s[N].　

Now, Dr lee gives you these N sub-strings: s[1],…s[N]. There might be several possibilities that the string S could be. For example, if Dr. lee gives you three sub-strings {“a”,“ab”,”ac”}, the string S could be “aabac”,”aacab”,”abaac”,…　

Your task is to output the lexicographically smallest S.