题1

字符串中第一个只出现一次的字符

class Solution {
public:
    char firstNotRepeatingChar(string s) {
        unordered_map<char, int> cnt;
        for (auto c : s) {
            cnt[c]++;
        }
        char res = '#';
        for (auto c : s) {
            if (cnt[c] == 1) {
                res = c;
                break;
            }
        }
        return res;
    }
};

题2

字符流中第一个只出现一次的字符

class Solution{
public:
    unordered_map<char, int> cnt;
    queue<char> q;
    //Insert one char from stringstream
    void insert(char ch){
        cnt[ch]++;
        if (cnt[ch] > 1) {
            while (q.size() && cnt[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();
    }
};