思路1：在字符流中，我们只记录有贡献的答案，即hash值为1的，大于1的全部舍弃

代码：

class Solution{
public:
    //Insert one char from stringstream
    queue<char> q;
    unordered_map<char,int> hash;
    void insert(char ch){
        hash[ch]++;
        if(hash[ch]>1){
            while(q.size()&&hash[q.front()]>1) q.pop();
        }else q.push(ch);
    }
    //return the first appearence once char in current stringstream
    char firstAppearingOnce(){
        if(q.empty())return '#';
        return q.front();
   
    }
};

思路2：暴力

代码：

class Solution{
public:
    //Insert one char from stringstream
    string s;
    unordered_map<char,int> hash;
    void insert(char ch){
        hash[ch]++;
        s+=ch;
    }
    //return the first appearence once char in current stringstream
    char firstAppearingOnce(){
        for(auto x:s) if(hash[x]==1) return x;
        return '#';
    }
};