字符流中第一个只出现一次的字符

题目：请实现一个函数用来找出字符流中第一个只出现一次的字符。例如，当从字符流中只读出前两个字符"go"时，第一个只出现一次的字符是'g'。当从该字符流中读出前六个字符"google"时，第一个只出现一次的字符是'l'。

//哈希表
class CharStatistics
{
public:
	CharStatistics() :index(0)
	{
		for (int i = 0;i < 256;++i)
			occurrence[i] = -1;
	}
	void insert(char ch)
	{
		if (occurrence[ch] == -1)
			occurrence[ch] = index;
		else if (occurrence[ch] >= 0)
			occurrence[ch] = -2;
		++index;
	}
	char FirstAppearingOnce()
	{
		char ch = '\0';
		int minIndex = numeric_limits<int>::max();
		for (int i = 0;i < 256;++i)
		{
			if (occurrence[i] >= 0 && occurrence[i] < minIndex)
			{
				ch = (char)i;
				minIndex = occurrence[i];
			}
		}
		return ch;
	}
private:
	int occurrence[256];
	int index;
};