利用字典树查找唯一标记某单词的最小前缀

class TireNode
{
public:
	bool isString;
	unordered_map<char, shared_ptr<TireNode>> m;
	TireNode(bool b) :isString(b){}
};
class Tire
{
public:
	shared_ptr<TireNode> root;

	Tire() :root(new TireNode(false)){}//重要
	bool insert(const string &s)
	{
		shared_ptr<TireNode> cur = root;
		for (int i = 0; i < s.size(); i++)
		{			
			char c = s[i];
			if (cur->m.find(c) == cur->m.end())
			{
				shared_ptr<TireNode> tmp(new TireNode(false));
				cur->m[c] = tmp;
			}
			cur = cur->m[c];
		}
		if (cur->isString)
			return false;//该字符串已经存在
		else
		{
			cur->isString = true;
			return true;
		}		
	}

	string get_shortest_prefix(const string &s)
	{
		if (s.empty())
			return "";
		string res;
		shared_ptr<TireNode> cur = root;
		for (int i = 0; i < s.size(); i++)
		{
			string tmp(1, s[i]);
			res += tmp;
			if (cur->m.find(s[i]) == cur->m.end())
				return res;
			else
				cur = cur->m[s[i]];
		}
		return "";
	}

};

string Find_shortest_prefix(const unordered_set<string> &D, const string &s)
{
	if (s.empty() || D.empty())
		return "";
	Tire T;
	for (unordered_set<string>::iterator i = D.begin(); i != D.end(); i++)
		T.insert(*i);
	return T.get_shortest_prefix(s);
}

//测试代码
string ss[] = { "dog", "be", "cut", "car" };
unordered_set<string> D;
for (int i = 0; i < 3; i++)
	D.emplace(ss[i]);//插入
cout << Find_shortest_prefix(D, "cat");