求字典序最小的拼接字符串之暴力枚举法

string subStr(int n, string str[]){
	sort(str, str+n);
	string ret = "";
	do
	{
		string tmp;
		for (int i = 0; i < n; ++i)
			tmp += str[i];
		if (ret == "" || tmp < ret)
			ret = tmp;
	} while(next_permutation(str, str+n));
	return ret;
}