使用一个hash和字符串来记录过程
class Solution
{
public:
map<char, int>M;
string s="";
//Insert one char from stringstream
void Insert(char ch)
{
s += ch;
M[ch]++;
}
//return the first appearence once char in current stringstream
char FirstAppearingOnce()
{
for (int i = 0; i < s.size();i++)
if (M[s[i]] == 1)return s[i];
return '#';
}
};