字符流中第一个不重复的字符

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"。

输出描述:

如果当前字符流没有存在出现一次的字符，返回#字符。

分析：
使用哈希表

class Solution
{
public:
    Solution() : 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] = -1;
        index++;
    }

    char FirstAppearingOnce()
    {
        char ch = '#';
        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:
    // occurrence[i] : A character with ASCII value i;
    // occurrence[i] = -1 : The character has not found;
    // occurrence[i] = -2 : The character has been found for mutlple times;
    // occurrence[i] >= 0 : The character has been found only once

    int occurrence[256];
    int index;
};