字符串处理 哈希表。

#include <unordered_map>
class Solution
{
public:
  //Insert one char from stringstream
    string s;
    unordered_map<char, int> mp;

    void Insert(char ch) {
         s += ch;
         mp[ch]++;
    }
  //return the first appearence once char in current stringstream
    char FirstAppearingOnce() {
        for(int i = 0; i < s.length(); i++){
            if(mp[s[i]] == 1){
                return s[i];
            }
        }
        return '#';
    }

};